# EDGAR Filing Document

**Accession Number:** 0001326190
**File Stem:** 0001326190-25-000066
**Filing Date:** 2025-11
**Character Count:** 126754
**Document Hash:** 974bdf6517d961f0fe7bdb5848b998d7
**Contains OCR:** False
**Source Format:** 

## Filing Content

## Filing Summary
**0001326190-25-000066.hdr.sgml**: 20251106

**ACCESSION NUMBER**: 0001326190-25-000066

**CONFORMED SUBMISSION TYPE**: 424B5

**PUBLIC DOCUMENT COUNT**: 6

**FILED AS OF DATE**: 20251106

**DATE AS OF CHANGE**: 20251106

**FILER**: 

**COMPANY DATA:**
- **COMPANY CONFORMED NAME:** Altimmune, Inc.
- **CENTRAL INDEX KEY:** 0001326190
- **STANDARD INDUSTRIAL CLASSIFICATION:** PHARMACEUTICAL PREPARATIONS [2834]
- **ORGANIZATION NAME:** 03 Life Sciences
- **EIN:** 202726770
- **STATE OF INCORPORATION:** DE
- **FISCAL YEAR END:** 1231

**FILING VALUES:**
- **FORM TYPE:** 424B5
- **SEC ACT:** 1933 Act
- **SEC FILE NUMBER:** 333-285355
- **FILM NUMBER:** 251458791

**BUSINESS ADDRESS:**
- **STREET 1:** 910 CLOPPER ROAD
- **STREET 2:** SUITE 201S
- **CITY:** GAITHERSBURG
- **STATE:** MD
- **ZIP:** 20878
- **BUSINESS PHONE:** 2406541450

**MAIL ADDRESS:**
- **STREET 1:** 910 CLOPPER ROAD
- **STREET 2:** SUITE 201S
- **CITY:** GAITHERSBURG
- **STATE:** MD
- **ZIP:** 20878

**FORMER COMPANY:**
- **FORMER CONFORMED NAME:** PHARMATHENE, INC
- **DATE OF NAME CHANGE:** 20071016

**FORMER COMPANY:**
- **FORMER CONFORMED NAME:** HEALTHCARE ACQUISITION CORP
- **DATE OF NAME CHANGE:** 20050505

[**Table of Contents**](#TOC)

Filed Pursuant to Rule 424(b)(5)

Registration No. 333-285355

**PROSPECTUS SUPPLEMENT**

**(To Prospectus dated March 13, 2025)**

![Graphic](alt-20251106x424b5001.jpg)

**Up to $200,000,000**

**Common Stock**

We have entered into an equity distribution agreement, dated November 6, 2025, or the Equity Distribution Agreement, with Leerink Partners LLC, or the sales agent, relating to shares of our common stock, par value $0.0001 per share, offered by this prospectus supplement. In accordance with the terms of the Equity Distribution Agreement, we may offer and sell shares of our common stock having an aggregate offering price of up to $200.0 million from time to time through the sales agent, pursuant to this prospectus supplement.

Our common stock is currently listed on The Nasdaq Global Market under the symbol "ALT." On November 5, 2025, the last reported sale price of our common stock was $3.75 per share.

Sales of our common stock, if any, under this prospectus supplement will be made in sales deemed to be "at the market offerings" as defined in Rule 415(a)(4) promulgated under the Securities Act of 1933, as amended, or the Securities Act. The sales agent is not required to sell any specific amount of securities, but will act as our sales agent using commercially reasonable efforts consistent with their normal trading and sales practices, on mutually agreed terms between the sales agent and us. There is no arrangement for funds to be received in any escrow, trust or similar arrangement.

The sales agent will be entitled to compensation at a commission rate of up to 3.0% of the aggregate gross sales sold thereunder. In connection with its sale of common stock on our behalf, the sales agent will be deemed to be an "underwriter" within the meaning of the Securities Act, and the compensation of the sales agent will be deemed to be underwriting commissions or discounts.

------

**Investing in our common stock involves a high degree of risk. You should review carefully the risks and uncertainties described under the heading "**[**Risk Factors**](#RISKFACTORS_856197)**" on page S-5 of this prospectus supplement, and under similar headings in the documents that are incorporated by reference into this prospectus supplement and the accompanying prospectus.**

------

**Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities, or passed upon the adequacy or accuracy of this prospectus supplement and the accompanying prospectus. Any representation to the contrary is a criminal offense.**

------

**Leerink Partners**

The date of this prospectus supplement is November 6, 2025.

------

[**Table of Contents**](#TOC)

#### **TABLE OF CONTENTS**

#### PROSPECTUS SUPPLEMENT

---

| | |
|:---|:---|
| [ABOUT THIS PROSPECTUS SUPPLEMENT](#ABOUTTHISPROSPECTUSSUPPLEMENT_741542) | S-1 |
| [PROSPECTUS SUPPLEMENT SUMMARY](#PROSPECTUSSUPPLEMENTSUMMARY) | S-3 |
| [THE OFFERING](#THEOFFERING) | S-4 |
| [RISK FACTORS](#RISKFACTORS_856197) | S-5 |
| [SPECIAL STATEMENT REGARDING FORWARD-LOOKING STATEMENTS](#SPECIALNOTEREGARDINGFORWARDLOOKINGSTATEM) | S-7 |
| [USE OF PROCEEDS](#USEOFPROCEEDS_93212) | S-9 |
| [DILUTION](#DILUTION_516257) | S-10 |
| [PLAN OF DISTRIBUTION](#Pro_supp_PlanOfDistribution) | S-11 |
| [LEGAL MATTERS](#LEGALMATTERS_744486) | S-12 |
| [EXPERTS](#EXPERTS_699535) | S-13 |
| [WHERE YOU CAN FIND MORE INFORMATION](#WHEREYOUCANFINDMOREINFORMATION_698614) | S-14 |
| [INCORPORATION OF DOCUMENTS BY REFERENCE](#INCORPORATIONOFDOCUMENTSBYREFERENCE_4377) | S-15 |
| **PROSPECTUS** |  |
| [ABOUT THIS PROSPECTUS](#ABOUTTHIS_281017) | 1 |
| [RISK FACTORS](#RISK_755360) | 2 |
| [CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS](#SPECIALNOTEREGARD_338512) | 3 |
| [OUR COMPANY](#OUR_COMPANY_PROSP) | 4 |
| [USE OF PROCEEDS](#USEOF_375312) | 6 |
| [SECURITIES WE MAY OFFER](#SECURITIESWEMAYSELL) | 7 |
| [DESCRIPTION OF CAPITAL STOCK](#DESCRIPTIONOFCAPITALSTOCK_PROSP) | 8 |
| [DESCRIPTION OF DEBT SECURITIES](#DESCRIPTIONOFDEBTSECURITIES_PROSP) | 12 |
| [DESCRIPTION OF WARRANTS](#DESCRIPTIONOFWARRANTS_PROSP) | 18 |
| [DESCRIPTION OF UNITS](#DESCRIPTIONOFUNITS_PROSP) | 19 |
| [PLAN OF DISTRIBUTION](#PLANOFDISTRIBUTION_PROSP) | 22 |
| [LEGAL MATTERS](#LEGAL_784747) | 25 |
| [EXPERTS](#EXP_372498) | 25 |
| [WHERE YOU CAN FIND MORE INFORMATION](#WHEREYOU_668710) | 26 |
| [INCORPORATION BY REFERENCE](#INCORPORATI_88026) | 27 |

---

S-i

------

[**Table of Contents**](#TOC)

#### ABOUT THIS PROSPECTUS SUPPLEMENT
This document is part of a "shelf" registration statement on Form S-3 that we filed with the SEC and is in two parts. The first part, the accompanying prospectus dated February 27, 2025, including the documents incorporated by reference into it, provides more general information. Generally, when we refer to this "prospectus," we are referring to both parts of this document combined. The second part is this prospectus supplement, which describes the specific terms of this offering of shares of common stock and also adds to and updates information contained in the accompanying prospectus and the documents incorporated by reference into this prospectus supplement and the accompanying prospectus.

This prospectus supplement relates only to an offering of up to $200.0 million of shares of our common stock through the sales agent. These sales, if any, will be made pursuant to the terms of the Equity Distribution Agreement, a copy of which is filed as an exhibit to the registration statement of which this prospectus supplement forms a part and is incorporated by reference into this prospectus supplement.

Before you invest, you should carefully read this prospectus supplement, the accompanying prospectus, all information incorporated by reference herein and the additional information described under "Where You Can Find More Information" and "Incorporation of Documents by Reference". These documents contain information you should consider when making your investment decision. To the extent that any statement that we make in this prospectus supplement is inconsistent with statements made in the accompanying prospectus or in any documents incorporated by reference, the statements made in this prospectus supplement will be deemed to modify or supersede those made in such documents; however, if any statement in one of these documents is inconsistent with a statement in another document having a later date and that is incorporated by reference herein, the statement in the document having the later date modifies or supersedes the earlier statement.

You should rely only on the information contained or incorporated by reference in this prospectus supplement, the accompanying prospectus, the documents incorporated by reference herein and any free writing prospectus we provide you. We have not, and the sales agent is not, authorized anyone to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it. We are not, and the sales agent is not, making an offer to sell these securities in any jurisdiction where the offer or sale is not permitted. You should assume that the information appearing in this prospectus supplement, the accompanying prospectus, the documents incorporated by reference herein and any free writing prospectus we provide you is accurate only as of the date on those respective documents. Our business, financial condition, results of operations and prospects may have changed since those dates. You should read this prospectus supplement, the accompanying prospectus, including the documents incorporated by reference herein, when making your investment decision.

The distribution of this prospectus supplement and the accompanying prospectus and the offering of the common stock in certain jurisdictions may be restricted by law. Persons outside the United States, or the U.S., who come into possession of this prospectus supplement and the accompanying prospectus must inform themselves about, and observe any restrictions relating to, the offering of the common stock and the distribution of this prospectus supplement and the accompanying prospectus outside the U.S. This prospectus supplement and the accompanying prospectus does not constitute, and may not be used in connection with, an offer to sell, or a solicitation of an offer to buy, any securities offered by this prospectus supplement and the accompanying prospectus by any person in any jurisdiction in which it is unlawful for such person to make such an offer or solicitation.

Unless otherwise indicated, information contained in this prospectus supplement, the accompanying prospectus or the documents incorporated by reference herein concerning our industry and the markets in which we operate, including our general expectations and market position, market opportunity and market share, is based on information from our own management estimates and research, as well as from industry and general publications and research, surveys and studies conducted by third parties. Management estimates are derived from publicly available information, our knowledge of our industry and assumptions based on such information and knowledge, which we believe to be reasonable. In addition, assumptions and estimates of our and our industry's future performance are necessarily subject to a high degree of uncertainty and risk due to a variety of factors, including those described in "Risk Factors" in this prospectus supplement, and the accompanying prospectus and in our most recent Annual Report, as filed with the SEC, as amended, supplemented or superseded from time to time by other reports we file with the SEC in the future, which are

------

[**Table of Contents**](#TOC)

incorporated by reference into this prospectus supplement. These and other important factors could cause our future performance to differ materially from our assumptions and estimates. See "Special Note Regarding Forward-Looking Statements."

"Altimmune," our logo and other trademarks, trade names or service marks of ours appearing in this prospectus supplement are our property. The other trademarks, trade names and service marks appearing in this prospectus supplement are the property of their respective owners. We do not intend our use or display of other companies' trademarks, trade names or service marks to imply an endorsement or sponsorship of us by such companies, or any relationship with such companies. Solely for convenience, trademarks and trade names referred to in this prospectus supplement may appear without the <sup>®</sup> or <sup>TM</sup> symbol.

Unless otherwise indicated or the context otherwise requires, all references in this prospectus supplement to "we," "us," "our," "Altimmune," the "Company" and similar designations refer to Altimmune, Inc., together with its subsidiaries. General information about us can be found on our website at www.altimmune.com. The information contained on, or that can be accessed through, our website is not a part of this prospectus supplement. We have included our website address in this prospectus supplement solely as an inactive textual reference.

------

[**Table of Contents**](#TOC)

#### PROSPECTUS SUPPLEMENT SUMMARY
*This summary highlights certain information about us, this offering and selected information contained elsewhere in or incorporated by reference into this prospectus supplement and the accompanying prospectus. This summary is not complete and does not contain all of the information that you should consider before deciding whether to invest in our common stock. For a more complete understanding of our company and this offering, we encourage you to read and consider carefully the more detailed information in this prospectus supplement and the accompanying prospectus, including the information incorporated by reference into this prospectus supplement and the accompanying prospectus, and the information referred to under the heading "*[*Risk Factors*](#RISKFACTORS_ProSupplement)*" in this prospectus supplement on page S-5 and in the documents incorporated by reference into this prospectus supplement and the accompanying prospectus.*

**Our Business**

**Overview**

Altimmune, Inc. is a late clinical-stage biopharmaceutical company developing novel peptide-based therapeutics for liver and cardiometabolic diseases. Our lead program is pemvidutide (formerly known as ALT-801), a balanced 1:1 glucagon/GLP-1 dual receptor agonist for the treatment of metabolic associated steatohepatitis ("MASH"), alcohol use disorder ("AUD") and alcohol-associated liver disease ("ALD"). We may also pursue additional indications for pemvidutide that leverage our differentiated clinical profile.

**Corporate History**

We were incorporated in Delaware in April 2005 and subsequently changed our name to Altimmune, Inc. in May 2017 upon the completion of our merger with PharmAthene, Inc. Our principal executive offices are located at 910 Clopper Road, Suite 201S, Gaithersburg, Maryland 20878, and our telephone number is (240) 654-1450. Our Internet website is www.altimmune.com and our investor relations website is located under the "Investors" tab. The information contained on, or that can be accessed through, our website is not a part of this prospectus supplement. We have included our website address in this prospectus supplement solely as an inactive textual reference.

------

[**Table of Contents**](#TOC)

#### THE OFFERING

---

| | |
|:---|:---|
| **Common stock offered by us** | Shares of our common stock having an aggregate offering price of up to $200.0 million.<br>|
| **Common stock outstanding immediately following the offering** | 157,675,718 shares, assuming sales of 53,333,333 shares of our common stock in this offering at an offering price of $3.75 per share, which was the last reported sale price of our common stock on The Nasdaq Global Market on November 5, 2025. The actual number of shares issued will vary depending on how many shares of our common stock we choose to sell and the prices at which such sales occur.<br>|
| **Plan of Distribution** | "At the market offering" that may be made from time to time through the sales agent. See "Plan of Distribution" on page S-11 of this prospectus supplement.<br>|
| **Use of Proceeds** | Our management will retain broad discretion regarding the allocation and use of the net proceeds. We intend to use the net proceeds from this offering for working capital and general corporate purposes. See "[*Use of Proceeds*](#USEOFPROCEEDS_93212)" for additional information. |
| **Risk Factors** | Investing in our securities involves risks. See "*Risk Factors*" beginning on page S-5 of this prospectus supplement and other information included or incorporated by reference into this prospectus supplement and the accompanying prospectus for a discussion of the factors you should carefully consider before deciding to invest in our securities.<br>|
| **Nasdaq Global Market symbol**  | "ALT" |

---

The number of shares of common stock to be outstanding after the offering is based on 95,598,665 shares outstanding as of September 30, 2025 and includes 8,743,720 shares of common stock issued through our at-the-market offering program from September 30, 2025 to November 5, 2025, and excludes:

● 8,661,357 shares of our common stock issuable upon the exercise of stock options outstanding as of September 30, 2025, at a weighted average exercise price of $8.24 per share;

● 1,135,950 shares of our common stock issuable upon the vesting of restricted stock units outstanding as of September 30, 2025;

● 4,301,248 shares reserved for issuance under our 2017 Omnibus Incentive Plan and 2018 Inducement Grant Plan as of September 30, 2025; and

● 60,585 shares reserved for issuance under our 2019 Employee Stock Purchase Plan as of September 30, 2025.

Except as otherwise indicated, all information in this prospectus supplement assumes no exercise of stock options listed above and no issuance of shares available, or that may become available, for future issuance under our equity compensation plans after September 30, 2025.

------

[**Table of Contents**](#TOC)

#### RISK FACTORS
*Investing in our common stock involves a high degree of risk. Investors should carefully consider the risks described below and under the heading "Risk Factors" in our most recent Annual Report on Form 10-K, as well as other information in this prospectus supplement and the documents incorporated by reference herein before deciding whether to invest in our securities. Such risks and uncertainties and those discussed below are not the only ones facing us. Additional risks and uncertainties not presently known to us, or that we currently see as immaterial, may also harm our business. If any of these risks occur, our business, financial condition and operating results could be harmed, the trading price of our common stock could decline and you could lose part or all of your investment*.

#### Risks Related to This Offering
***We will have broad discretion in the use of the net proceeds from this offering and may not use them effectively.***

We currently intend to use the net proceeds of this offering for working capital and general corporate purposes, which may include capital expenditures, research and development expenditures, regulatory affairs expenditures, clinical trial expenditures, acquisitions of new technologies and investments, business combinations and the repayment, refinancing, redemption or repurchase of indebtedness or capital stock, as further described in the section of this prospectus supplement entitled "Use of Proceeds". We will have broad discretion in the application of the net proceeds in the category of other working capital and general corporate purposes and investors will be relying on the judgment of our management regarding the application of the proceeds of this offering*.*

The precise amount and timing of the application of these proceeds will depend upon a number of factors, such as the timing and progress of our research and development efforts, our funding requirements and the availability and costs of other funds. As of the date of this prospectus supplement, we cannot specify with certainty all of the particular uses for the net proceeds to us from this offering. Depending on the outcome of our efforts and other unforeseen events, our plans and priorities may change and we may apply the net proceeds of this offering in different manners than we currently anticipate.

The failure by our management to apply these funds effectively could harm our business, financial condition and results of operations and compromise our ability to pursue our growth strategy. Furthermore, we might not be able to yield a significant return, if any, on our investment of these net proceeds. You will not have the opportunity to influence our decisions on how to use the net proceeds from this offering. Pending their use, we may invest our cash, including the net proceeds from this offering, in short-term, interest bearing instruments or in a manner that does not produce income or that loses value.

***If you purchase our common stock in this offering, you will incur immediate and substantial dilution in the book value of your shares.***

The offering price per share in this offering may exceed the net tangible book value per share of our common stock outstanding prior to this offering. Assuming that an aggregate of 53,333,333 shares of our common stock are sold during the term of the Equity Distribution Agreement with the sales agent at a price of $3.75 per share, the last reported sale price of our common stock on The Nasdaq Global Market on November 5, 2025, for aggregate net proceeds of approximately $196.8 million, after deducting commissions and estimated aggregate offering expenses payable by us, you will experience immediate dilution of $1.12 per share, representing the difference between our as adjusted net tangible book value per share as of September 30, 2025 after giving effect to this offering and the assumed offering price. The exercise of outstanding stock options may result in further dilution of your investment. See the section entitled "Dilution" below for a more detailed illustration of the dilution you would incur if you participate in this offering.

***The actual number of shares we will issue under the Equity Distribution Agreement with the sales agent, at any one time or in total, is uncertain.***

Subject to certain limitations in the Equity Distribution Agreement with the sales agent and compliance with applicable law, we have the discretion to deliver placement notices to the sales agent at any time throughout the term of the Equity Distribution Agreement. The number of shares that are sold by the sales agent after delivering a placement

------

[**Table of Contents**](#TOC)

notice will fluctuate based on the market price of the common stock during the sales period and limits we set with the sales agent.

***You may experience future dilution as a result of future equity offerings.***

In order to raise additional capital, we may in the future offer additional shares of our common stock or other securities convertible into or exchangeable for our common stock at prices that may not be the same as the price per share in this offering. We may sell shares or other securities in any other offering at a price per share that is less than the price per share paid by investors in this offering, and investors purchasing shares or other securities in the future could have rights superior to existing stockholders. The price per share at which we sell additional shares of our common stock, or securities convertible or exchangeable into common stock, in future transactions may be higher or lower than the price per share paid by investors in this offering.

***We do not intend to pay dividends in the foreseeable future.***

We have never paid cash dividends on our common stock. We currently intend to retain our future earnings, if any, to finance the operation and growth of our business and currently do not plan to pay any cash dividends in the foreseeable future.

***The common stock offered hereby will be sold in "at the market offerings," and investors who buy shares at different times will likely pay different prices.***

Investors who purchase shares in this offering at different times will likely pay different prices, and so may experience different outcomes in their investment results. We will have discretion, subject to market demand, to vary the timing, prices, and quantity of shares sold, and there is no minimum or maximum sales price. Investors may experience a decline in the value of their shares as a result of share sales made at prices lower than the prices they paid.

------

[**Table of Contents**](#TOC)

#### SPECIAL STATEMENT REGARDING FORWARD-LOOKING STATEMENTS
This prospectus supplement and the accompanying prospectus and the documents incorporated by reference into the prospectus supplement and the accompanying prospectus contain "forward-looking statements" by us within the meaning of Section 27A of the Securities Act of 1933, as amended, or the Securities Act, and Section 21E of the Securities Exchange Act of 1934, as amended, or the Exchange Act, including, without limitation, statements as to expectations, beliefs and strategies regarding the future. These statements involve known and unknown risks, uncertainties and other important factors that may cause our actual results, performance or achievements to be materially different from any future results, performance or achievements expressed or implied by the forward-looking statements. These forward-looking statements rely on a number of assumptions concerning future events and include statements relating to:

● our ability to develop and commercialize our current and future product candidates;

● our ability to expand our pipeline of product candidates and the success of future product candidate advancements, including the success of future clinical trials, and our ability to commercialize our products;

● the reliability of the results of the studies relating to human safety and possible adverse effects resulting from the administration of our new product candidates;

● our ability to obtain potential regulatory approvals on the timelines anticipated, or at all;

● our ability to obtain additional patents or extend existing patents on the timelines anticipated, or at all;

● our ability to identify and consummate potential future strategic partnerships or business combinations;

● our expectations regarding the potential market size and the size of the patient populations for our product candidates, if approved or commercial use;

● our anticipated financial or operational results;

● our ability to obtain additional capital resources;

● breaches of data privacy, or disruptions in our information technology systems;

● our ability to continue to satisfy the listing requirements of The Nasdaq Global Market; and

● our use of proceeds from this offering.

Any forward-looking statements should be considered in light of these factors. Words such as "anticipates," "believes," "forecasts," "potential," "goal," "contemplates," "expects," "intends," "plans," "projects," "hopes," "seeks," "estimates," "strategy," "target," "continues," "ongoing," "opportunity," "could," "would," "should," "guidance," "likely," "usually," "will," "may," "can," "designed to," "potential," "future," "foreseeable future" and similar expressions and variations, and negatives of these words, identify forward-looking statements. These forward-looking statements are based on the expectations, estimates, projections, beliefs and assumptions of our management based on information currently available to management, all of which are subject to change. These forward-looking statements are not guarantees of future performance and are subject to risks and uncertainties that could cause actual results to differ materially from the results contemplated by the forward-looking statements. Many of the important factors that will determine these results and values are beyond our ability to control or predict. You are cautioned not to put undue reliance on any forward-looking statements. Except as otherwise required by law, we do not assume any obligation to update any forward-looking statements.

In evaluating an investment in shares of our common stock, you should carefully consider the discussion of risks and uncertainties described under the heading "*Risk Factors*" contained in this prospectus supplement, and under similar headings in other documents including in other filings with the SEC, which may be amended, supplemented or superseded from time to time by other reports we file with the SEC in the future, that are incorporated by reference in this prospectus supplement. You should carefully read this prospectus supplement together with the information incorporated by reference in this prospectus supplement as described under the headings "*Where You Can Find More Information*" and "*Incorporation of Documents by Reference*" completely and with the understanding that our actual future results may be materially different from what we expect.

All subsequent written and oral forward-looking statements attributable to us or persons acting on our behalf are expressly qualified in their entirety by our cautionary statements. The forward-looking statements included or incorporated by reference herein are made only as of the date of this prospectus supplement (or as of the date of any such

------

[**Table of Contents**](#TOC)

document incorporated by reference). We do not intend, and undertake no obligation, to update these forward-looking statements, except as required by law.

------

[**Table of Contents**](#TOC)

#### USE OF PROCEEDS
We may issue and sell our common stock having aggregate sales proceeds of up to $200.0 million from time to time. Because there is no minimum offering amount required as a condition to close this offering, the actual total public offering amount, commissions and proceeds to us, if any, are not determinable at this time. There can be no assurance that we will sell any shares under or fully utilize the Equity Distribution Agreement as a source of financing.

We currently intend to use the net proceeds from this offering for working capital and general corporate purposes, which may include capital expenditures, research and development expenditures, regulatory affairs expenditures, clinical trial expenditures, acquisitions of new technologies and investments, business combinations and the repayment, refinancing, redemption or repurchase of indebtedness or capital stock.

The precise amount and timing of the application of these proceeds will depend upon a number of factors, such as the timing and progress of our research and development efforts, our funding requirements and the availability and costs of other funds. As of the date of this prospectus supplement, we cannot specify with certainty all of the particular uses for the net proceeds to us from this offering. Accordingly, our management will have broad discretion in the timing and application of these proceeds. Pending application of the net proceeds as described above, we intend to temporarily invest the proceeds in short-term, interest-bearing instruments.

------

[**Table of Contents**](#TOC)

#### DILUTION
If you purchase our common stock in this offering, your interest will be diluted to the extent of the difference between the public offering price per share and the net tangible book value per share of our common stock after this offering. We calculate net tangible book value per share by dividing our net tangible assets (tangible assets less total liabilities) by the number of shares of our common stock issued and outstanding as of September 30, 2025.

Our net tangible book value as of September 30, 2025 was approximately $184.1 million, or $1.93 per share. After giving effect to our issuance and sale of an aggregate of 8,743,720 shares of common stock for net proceeds of $33.8 million through our at-the-market offerings program from September 30 to November 5, 2025, our adjusted net tangible book value as of September 30, 2025 would have been $217.9 million or $2.09 per share of common stock.

After giving effect to the assumed sale of our common stock during the term of the Equity Distribution Agreement with the sales agent in the aggregate amount of $200.0 million at an assumed offering price of $3.75 per share, the last reported sale price of our common stock on The Nasdaq Global Market on November 5, 2025, and after deducting commissions and estimated aggregate offering expenses payable by us, our adjusted, as adjusted, net tangible book value as of September 30, 2025 would have been approximately $414.7 million, or $2.63 per share of common stock. This represents an immediate increase in the net tangible book value of $0.54 per share to our existing stockholders and an immediate dilution in net tangible book value of $1.12 per share to new investors. The following table illustrates this per share dilution:

---

| | | |
|:---|:---|:---|
| Assumed public offering price per share  |  | $3.75 |
| Adjusted net tangible book value per share as of September 30, 2025 | $2.09 |  |
| Increase in adjusted net tangible book value per share attributable to this offering | $0.54 |  |
| As adjusted net tangible book value per share as of September 30, 2025, after giving effect to this offering |  | 2.63 |
| Dilution per share to new investors purchasing shares in this offering |  | $1.12 |

---

The number of shares of common stock to be outstanding after the offering is based on 95,598,665 shares outstanding as of September 30, 2025 and includes 8,743,720 shares of common stock issued through our at-the-market offering program from September 30 to November 5, 2025, and exclude the following:

● 8,661,357 shares of our common stock issuable upon the exercise of stock options outstanding as of September 30, 2025, at a weighted average exercise price of $8.24 per share;

● 1,135,950 shares of our common stock issuable upon the vesting of restricted stock units outstanding as of September 30, 2025:

● 4,301,248 shares reserved for issuance under our 2017 Omnibus Incentive Plan and 2018 Inducement Grant Plan as of September 30, 2025; and

● 60,585 shares reserved for issuance under our 2019 Employee Stock Purchase Plan as of September 30, 2025.

Except as otherwise indicated, all information in this prospectus supplement assumes no exercise of stock options listed above and no issuance of shares available, or that may become available, for future issuance under our equity compensation plans after September 30, 2025.

To the extent that options outstanding as of September 30, 2025 have been or are exercised, or other shares are issued, investors purchasing shares in this offering could experience further dilution. In addition, we may choose to raise additional capital due to market conditions or strategic considerations, even if we believe we have sufficient funds for our current or future operating plans. To the extent that additional capital is raised through the sale of equity or convertible debt securities, the issuance of these securities could result in further dilution to our stockholders.

------

[**Table of Contents**](#TOC)

#### PLAN OF DISTRIBUTION
We have entered into an Equity Distribution Agreement with Leerink Partners LLC as our sales agent under which we may offer and sell shares of our common stock from time to time. Pursuant to this prospectus supplement, we may offer and sell shares having an aggregate gross offering price of up to $200.0 million. The sales agent will use commercially reasonable efforts to sell on our behalf all of the shares of our common stock requested to be sold by us, consistent with their normal trading and sales practices, under the terms and subject to the conditions set forth in the Equity Distribution Agreement. We may instruct the sales agent not to sell our common stock if the sales cannot be effected at or above the price designated by us in any instruction. We or the sales agent may suspend the offering of our common stock upon proper notice and subject to other conditions, as further described in the Equity Distribution Agreement. A copy of the Equity Distribution Agreement is filed as an exhibit to the Company's Form 8-K filed with the Securities and Exchange Commission on November 6, 2025.

The sales agent may sell our common stock by any method permitted by law deemed to be an "at the market offering" as defined in Rule 415(a)(4) promulgated under the Securities Act, including sales made directly on or through The Nasdaq Global Market or on any other existing trading market for our common stock. The sales agent will provide written confirmation to us following the close of trading on The Nasdaq Global Market each day in which our common stock is sold under the Equity Distribution Agreement. Each such confirmation will include the number of shares of our common stock sold on such day, the net proceeds to us and the compensation payable by us to the sales agent in connection with such sales.

We will pay the sales agent commissions for their services in acting as the sales agent in the sale of our common stock. The sales agent will be entitled to compensation in an amount of up to 3.0% of the gross sales price of all common stock sold through it as sales agent under the Equity Distribution Agreement. We have also agreed to reimburse the sales agent for the out-of-pocket reasonable fees and disbursements of its legal counsel, in an amount not to exceed $75,000, as well as certain ongoing expenses. We estimate that the total expenses for this offering, excluding compensation payable to the sales agent under the terms of the Equity Distribution Agreement, will be approximately $235,000.

Settlement for sales of our common stock will occur on the first business day following the date on which any such sales are made, or on some other date that is agreed upon by us and the sales agent in connection with a particular transaction, in return for payment of the net proceeds to us. There is no arrangement for funds to be received in an escrow, trust or similar arrangement.

We will report at least quarterly the number of shares of our common stock sold through the sales agent, under the Equity Distribution Agreement and the net proceeds to us in connection with such sales.

The sales agent and its affiliates have from time to time provided, and may in the future provide, various investment banking, commercial banking, fiduciary and advisory services for us for which they have received, and may in the future receive, customary fees and expenses. The sales agent and its affiliates may from time to time engage in other transactions with and perform services for us in the ordinary course of their business.

In connection with the sale of our common stock on our behalf, the sales agent will be deemed to be an underwriter within the meaning of the Securities Act, and the compensation paid by us to the sales agent will be deemed to be underwriting commissions or discounts. We have agreed to indemnify the sales agent against specified liabilities, including liabilities under the Securities Act, and to contribute to payments that the sales agent may be required to make because of such liabilities.

The offering of our common stock pursuant to the Equity Distribution Agreement will terminate upon the termination of the Equity Distribution Agreement. The Equity Distribution Agreement may be terminated by the sales agent or us at any time on the close of business on the date of receipt of written notice, and by the sales agent at any time in certain circumstances, including any suspension or limitation on the trading of our common stock on The Nasdaq Global Market, as further described therein.

------

[**Table of Contents**](#TOC)

#### LEGAL MATTERS
The validity of the issuance of the common stock offered by this prospectus supplement will be passed upon for us by Goodwin Procter LLP. Leerink Partners LLC is being represented in connection with this offering by Cooley LLP, New York, New York.

------

[**Table of Contents**](#TOC)

#### EXPERTS
The consolidated financial statements of Altimmune, Inc. appearing in Altimmune, Inc.'s Annual Report on Form 10-K for the year ended December 31, 2024, have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their report thereon, included therein, and incorporated herein by reference. Such consolidated financial statements are incorporated herein by reference in reliance upon such report given on the authority of such firm as experts in accounting and auditing.

------

[**Table of Contents**](#TOC)

#### WHERE YOU CAN FIND MORE INFORMATION
We are a reporting company and file annual, quarterly and current reports, proxy statements and other information with the SEC. We have filed with the SEC a registration statement on Form S-3 under the Securities Act with respect to the securities being offered under this prospectus supplement and the accompanying prospectus. This prospectus supplement and the accompanying prospectus do not contain all of the information set forth in the registration statement and the exhibits to the registration statement. For further information with respect to us and the securities being offered under this prospectus supplement and the accompanying prospectus, we refer you to the registration statement and the exhibits and schedules filed as a part of the registration statement. The SEC maintains an Internet site that contains reports, proxy and information statements, and other information regarding issuers that file electronically with the SEC, including Altimmune, Inc. The SEC's Internet site can be found at *www.sec.gov*.

------

[**Table of Contents**](#TOC)

#### INCORPORATION OF DOCUMENTS BY REFERENCE
The SEC allows us to "incorporate by reference" information into this prospectus supplement, which means that we can disclose important information to you by referring you to another document filed separately with the SEC. The documents incorporated by reference into this prospectus supplement contain important information that you should read about us.

The following documents are incorporated by reference into this prospectus supplement:

● our Annual Report on Form 10-K for the fiscal year ended December 31, 2024, filed with the SEC on February 27, 2025 (the " [2024 10-K"](https://www.sec.gov/Archives/edgar/data/1326190/000132619025000015/alt-20241231x10k.htm)) ;

● our Quarterly Reports on Form 10-Q for the quarterly period ended [March 31, 2025](https://www.sec.gov/Archives/edgar/data/1326190/000132619025000034/alt-20250331x10q.htm) , filed with the SEC on May 13, 2025; for the quarter ended [June 30, 2025](https://www.sec.gov/Archives/edgar/data/1326190/000132619025000040/alt-20250630x10q.htm) , filed with the SEC on August 12, 2025 and for the quarter ended [September 30, 2025](https://www.sec.gov/Archives/edgar/data/1326190/000132619025000064/alt-20250930x10q.htm) , filed with the SEC on November 6, 2025.

● the Company's Current Reports on Form 8-K filed with the SEC on [February 25, 2025](https://www.sec.gov/Archives/edgar/data/1326190/000132619025000003/alt-20250224x8k.htm) , [February 27, 2025](https://www.sec.gov/Archives/edgar/data/1326190/000132619025000011/alt-20250224x8k.htm) , [May 13, 2025](https://www.sec.gov/Archives/edgar/data/1326190/000132619025000031/alt-20250513x8k.htm) , [September 25, 2025](https://www.sec.gov/Archives/edgar/data/1326190/000132619025000050/alt-20250925x8k.htm) , [October 3, 2025](https://www.sec.gov/Archives/edgar/data/1326190/000132619025000053/alt-20251001x8k.htm) and [November 6, 2025](https://www.sec.gov/Archives/edgar/data/1326190/000132619025000055/alt-20251106xex10d1.htm) ; and

● the descriptions of our common stock set forth in the registration statement on Form 8-A registering our common stock under Section 12 of the Exchange Act, which was filed as [Exhibit 4.1](https://www.sec.gov/Archives/edgar/data/1326190/000132619025000015/alt-20241231xex4d1.htm) to our 2024 10-K, including any amendments or reports filed for purposes of updating such description .

We also incorporate by reference any future filings (other than current reports furnished under Item 2.02 or Item 7.01 of Form 8-K and exhibits filed on such form that are related to such items unless such Form 8-K expressly provides to the contrary) made with the SEC pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act, including those made after the date of the initial filing of the registration statement of which this prospectus supplement is a part and prior to effectiveness of such registration statement, until we file a post- effective amendment that indicates the termination of the offering of the securities made by this prospectus supplement and will become a part of this prospectus supplement from the respective dates that such documents are filed with the SEC. Any statement contained herein or in a document incorporated or deemed to be incorporated by reference herein shall be deemed to be modified or superseded for purposes hereof or of the related prospectus supplement to the extent that a statement contained herein or in any other subsequently filed document which is also incorporated or deemed to be incorporated herein modifies or supersedes such statement. Any such statement so modified or superseded shall not be deemed, except as so modified or superseded, to constitute a part of this prospectus supplement.

Documents incorporated by reference are available from us, without charge. You may obtain documents incorporated by reference in this prospectus supplement by requesting them in writing or by telephone at the following address:

Altimmune, Inc.

910 Clopper Road, Suite 201S <br>Gaithersburg, Maryland 20878

Attention: Chief Financial Officer

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(240) 654-1450

------

[**Table of Contents**](#TOC)

**PROSPECTUS**

**$400,000,000**

**Common Stock**

**Preferred Stock**

**Debt Securities**

**Warrants**

**Units**

------

We may from time to time issue, in one or more series or classes, up to $400,000,000 in aggregate principal amount of our common stock, preferred stock, debt securities, warrants and/or units. We may offer these securities separately or together in units. We will specify in the applicable accompanying prospectus supplement the terms of the securities being offered. We may sell these securities to or through underwriters and also to other purchasers or through agents. We will set forth the names of any underwriters or agents, and any fees, conversions or discount arrangements, in the applicable accompanying prospectus supplement. We may not sell any securities under this prospectus without delivery of the applicable prospectus supplement.

You should read this document and any prospectus supplement or amendment carefully before you invest in our securities.

Our common stock is listed on The Nasdaq Global Market under the symbol "ALT." On February 24, 2025, the closing price for our common stock, as reported on The Nasdaq Global Select Market, was $6.11 per share. Our principal executive office is located at 910 Clopper Road, Suite 201S, Gaithersburg, Maryland 20878.

**Investing in our securities involves a high degree of risk. You should review carefully the risks and uncertainties referenced under the heading "**[**Risk Factors**](#RISK_755360)**" contained in this prospectus beginning on page 2 and any applicable prospectus supplement, and under similar headings in the other documents that are incorporated by reference into this prospectus.**

**Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.**

**The date of this Prospectus is March 13, 2025.**

------

[**Table of Contents**](#TOC)

**TABLE OF CONTENTS**

---

| | |
|:---|:---|
|  | **Page** |
| [About This Prospectus](#ABOUTTHIS_281017) | 1 |
| [Risk Factors](#RISK_755360) | 2 |
| [Cautionary Statement Regarding Forward-Looking Statements](#SPECIALNOTEREGARD_338512) | 3 |
| [Our Company](#OUR_COMPANY_PROSP) | 4 |
| [Use of Proceeds](#USEOF_375312) | 6 |
| [Securities We May Offer](#SECURITIESWEMAYSELL) | 7 |
| [Description of Capital Stock](#DESCRIPTIONOFCAPITALSTOCK_PROSP) | 8 |
| [Description of Debt Securities](#DESCRIPTIONOFDEBTSECURITIES_PROSP) | 12 |
| [Description of Warrants](#DESCRIPTIONOFWARRANTS_PROSP) | 18 |
| [Description of Units](#DESCRIPTIONOFUNITS_PROSP) | 19 |
| [Plan of Distribution](#PLANOFDISTRIBUTION_PROSP) | 22 |
| [Legal Matters](#LEGAL_784747) | 25 |
| [Experts](#EXP_372498) | 25 |
| [Where You Can Find More Information](#WHEREYOU_668710) | 26 |
| [Incorporation by Reference](#INCORPORATI_88026) | 27 |

---

------

[**Table of Contents**](#TOC)

#### ABOUT THIS PROSPECTUS
This prospectus is part of a registration statement on Form S-3 that we filed with the Securities and Exchange Commission, or the SEC, utilizing a "shelf" registration process. Under the shelf registration, we may offer shares of our common stock and preferred stock, various series of warrants to purchase common stock or preferred stock, debt securities or any combination thereof, from time to time in one or more offerings.

This prospectus provides you with a general description of the securities we may offer. Each time we sell securities, we will provide one or more prospectus supplements that will contain specific information about the terms of the offering. The applicable prospectus supplement may also add, update or change information contained in this prospectus. You should read both this prospectus and any accompanying prospectus supplement together with the additional information described under the heading "Where You Can Find More Information" beginning on page 26 of this prospectus.

You should rely only on the information contained in or incorporated by reference in this prospectus, the accompanying prospectus supplement or in any related free writing prospectus filed by us with the SEC. We have not authorized anyone to provide you with different information. This prospectus and the accompanying prospectus supplement do not constitute an offer to sell or the solicitation of an offer to buy any securities other than the securities described in the accompanying prospectus supplement or an offer to sell or the solicitation of an offer to buy such securities in any circumstances in which such offer or solicitation is unlawful. You should assume that the information appearing in this prospectus, any prospectus supplement, the documents incorporated by reference and any related free writing prospectus is accurate only as of their respective dates. Our business, financial condition, results of operations and prospects may have changed materially since those dates.

Unless the context otherwise requires, references to "Altimmune," the "Company," "we," "us" and "our" refer to Altimmune, Inc., and its wholly owned, consolidated subsidiaries, or either or all of them as the context may require.

------

[**Table of Contents**](#TOC)

#### RISK FACTORS
Investing in our securities involves a high degree of risk. You should carefully consider the risks described in the documents incorporated by reference in this prospectus and any prospectus supplement, as well as other information we include or incorporate by reference into this prospectus and any applicable prospectus supplement, before making an investment decision. Our business, financial condition or results of operations could be materially and adversely affected by the materialization of any of these risks. The trading price of our securities could decline due to the materialization of any of these risks, and you may lose all or part of your investment. This prospectus and the documents incorporated herein by reference also contain forward-looking statements that involve risks and uncertainties. Actual results could differ materially from those anticipated in these forward-looking statements as a result of certain factors, including the risks described in the documents incorporated herein by reference, including (i) our most recent Annual Report on Form 10-K, which is on file with the SEC and is incorporated herein by reference, and (ii) other documents we file with the SEC that are deemed incorporated by reference into this prospectus, including our Quarterly Reports on Form 10-Q.

------

[**Table of Contents**](#TOC)

#### CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS
This prospectus, any applicable prospectus supplement, any related free writing prospectus, and the information incorporated by reference herein and therein contain express or implied statements that are based on our management's belief and assumptions and on information currently available to our management are considered forward-looking within the meaning of Section 27A of the Securities Act and Section 21E of the Exchange Act. All statements other than statements of historical facts are forward-looking statements, including statements regarding our future results of operations or of our financial position, business strategy, plans and objectives of management for future operations, or state other forward-looking information. In some cases, you can identify these statements by forward-looking words such as "anticipate," "believe," "contemplate," "continue," "could," "estimate," "expect," "intend," "may," "plan," "potential," "predict," "project," "should," "target," "will," or "would" or the negative of these words or other similar terms or expressions. We believe that it is important to communicate our future expectations to our investors. However, there may be events in the future that we are not able to accurately predict or control and that may cause our actual results to differ materially from the expectations we describe in our forward-looking statements.

These forward-looking statements are neither promises nor guarantees of future performance and relate to future events or our future operational or financial performance, and involve known and unknown risks, uncertainties, and other factors, many of which are beyond our control, that may cause our actual results, performance, or achievements to be materially different from any future results, performance, or achievements expressed or implied by these forward-looking statements, including, without limitation the risk factors and cautionary statements described in other documents that we file from time to time with the SEC, specifically under "Item 1A. Risk Factors" and elsewhere in our most recent Annual Report on Form 10-K and the section of any accompanying prospectus supplement entitled "Risk Factors," as may be updated by our subsequent Quarterly Reports on Form 10-Q and other filings we make with the SEC.

Given these uncertainties, you should not place undue reliance on these forward-looking statements. These forward-looking statements speak only as of the date on which the statements were made and are not guarantees of future performance. Except as may be required by applicable law, we do not undertake or intend to update any forward-looking statements after the date of this prospectus or the respective dates of documents incorporated by reference herein that include forward-looking statements.

------

[**Table of Contents**](#TOC)

#### OUR COMPANY
**Our Business**

***Overview***

Altimmune, Inc. is a clinical stage biopharmaceutical company focused on developing treatments for obesity, metabolic and liver diseases. Our lead product candidate, pemvidutide (formerly known as ALT-801), is a novel, investigational, peptide-based GLP-1/glucagon dual receptor agonist. Pemvidutide is currently in clinical development for obesity and metabolic associated steatohepatitis ("MASH"). We also expect to pursue additional indications for pemvidutide that leverage the differentiated clinical profile of pemvidutide.

*Pemvidutide*

Pemvidutide is a novel, investigational, peptide-based GLP-1/glucagon dual receptor agonist currently in clinical development for the treatment of obesity and MASH. Activation of the GLP-1 and glucagon receptors is believed to mimic the complementary effects of diet and exercise on weight loss, with GLP-1 suppressing appetite and glucagon increasing energy expenditure. Glucagon is also recognized as having direct effects on hepatic fat metabolism, which is believed to lead to rapid reductions in levels of liver fat and serum lipids.

**Pemvidutide mechanism of action: Optimized for obesity and MASH**

![Graphic](alt-20251106x424b5007.jpg)

Over 500 subjects have received pemvidutide to date in 6 completed clinical trials. In those trials pemvidutide appear safe and demonstrated what we believe to be a favorable safety profile, compelling weight loss, class-leading lean mass preservation, and robust reductions in triglycerides, low density lipoprotein ("LDL"), liver fat content, measures of liver inflammation, and blood pressure. Our clinical development has always focused on the comorbidities of obesity and MASH as these often drive the outcomes of these diseases. Based on the clinical data we have accumulated, we believe pemvidutide may be able to address the most common comorbidities associated with both obesity and MASH as shown below.

------

[**Table of Contents**](#TOC)

**U.S. prevalence of comorbidities in obesity and MASH**

![Graphic](alt-20251106x424b5008.jpg)

**Corporate History**

We were incorporated in Delaware in April 2005 and subsequently changed our name to Altimmune, Inc. in May 2017 upon the completion of our merger with PharmAthene, Inc. Our principal executive offices are located at 910 Clopper Road, Suite 201S, Gaithersburg, Maryland 20878, and our telephone number is (240) 654-1450. Our Internet website is www.altimmune.com and our investor relations website is located under the "Investors" tab. The information contained on, or that can be accessed through, our website is not a part of this prospectus. We have included our website address in this prospectus solely as an inactive textual reference.

------

[**Table of Contents**](#TOC)

#### USE OF PROCEEDS
We intend to use the net proceeds from the sale of any securities offered under this prospectus for general corporate purposes unless otherwise indicated in the applicable prospectus supplement. General corporate purposes may include, but are not limited to research and clinical development of current or additional pipeline candidates, working capital, capital expenditures and other general corporate purposes. We may temporarily invest the net proceeds in a variety of capital preservation instruments, including short-term, investment-grade, interest-bearing instruments and U.S. government securities, or may hold such proceeds as cash, until they are used for their stated purpose. We have not determined the amount of net proceeds to be used specifically for such purposes. As a result, management will retain broad discretion over the allocation of net proceeds.

------

[**Table of Contents**](#TOC)

#### SECURITIES WE MAY OFFER OR SELL
This prospectus contains summary descriptions of the securities we may offer or sell from time to time. These summary descriptions are not meant to be complete descriptions of each security. The particular terms of any security will be described in the applicable prospectus supplement. This prospectus provides you with a general description of the securities we may offer. Each time we sell securities described herein, we will provide prospective investors with a supplement to this prospectus that will contain specific information about the terms of that offering, including the specific amounts, prices and terms of the securities offered.

We may sell the securities to or through underwriters, dealers or agents, directly to purchasers or through a combination of any of these methods of sale or as otherwise set forth below under "Plan of Distribution." We, as well as any agents acting on our behalf, reserve the sole right to accept and to reject in whole or in part any proposed purchase of securities. Any prospectus supplement will set forth the names of any underwriters, dealers, agents or other entities involved in the sale of securities described in that prospectus supplement and any applicable fee, commission or discount arrangements with them.

------

[**Table of Contents**](#TOC)

#### DESCRIPTION OF CAPITAL STOCK
*The following description of our common stock and preferred stock, together with the additional information we include in any applicable prospectus supplements, summarizes the material terms and provisions of the common stock and preferred stock that we may offer under this prospectus. The following description of our capital stock does not purport to be complete and is subject to, and qualified in its entirety by, our certificate of incorporation and bylaws, which are exhibits to the registration statement of which this prospectus forms a part, and by applicable law. The terms of our common stock and preferred stock may also be affected by Delaware law.*

**General**

Under our charter, our authorized capital stock consists of two hundred million (200,000,000) shares of common stock, par value $0.0001 per share, and one million (1,000,000) shares of undesignated preferred stock, par value $0.0001 per share.

**Common Stock**

Under our Amended and Restated Certificate of Incorporation, as amended, to which we refer as our "charter," we are currently authorized to issue 200,000,000 shares of common stock, par value $0.0001 per share. As of February 24, 2025, we had 77,013,658 shares of common stock outstanding.

Holders of our common stock are entitled to one vote for each share of common stock held of record on all matters to be voted on by stockholders, except as otherwise provided by law or in any preferred stock designation. Our bylaws specify that, except as otherwise required by law or our charter, the presence in person or by proxy of holders of a majority of the shares entitled to vote at a meeting of stockholders will be necessary, and will constitute a quorum, for the transaction of business at such meeting. Our bylaws furthermore specify that all elections of directors will be determined by a plurality of the votes and that, except as otherwise provided by law or in the charter or bylaws, any other matter will be determined by the vote of a majority of the shares which are voted with regard to it. Holders of our common stock have no conversion, preemptive or other subscription rights and there are no sinking fund or redemption provisions applicable to the common stock.

There is no cumulative voting with respect to the election of directors, with the result that the holders of more than 50% of the shares voting for the election of directors can elect all of the directors then up for election. Holders of our common stock are entitled to receive dividends when, as and if declared by our board of directors out of funds legally available therefor. In the event of our liquidation, dissolution or winding up, the holders of common stock are entitled to share in all assets remaining which are available for distribution to them after payment of liabilities and after provision has been made for each class of stock, if any, having preference over the common stock.

Our common stock is listed on The Nasdaq Global Select Market under the trading symbol "ALT".

***Annual Meeting***

Annual meetings of our stockholders are held on the date designated in accordance with our amended and restated by-laws. Written notice must be mailed to each stockholder entitled to vote not less than ten nor more than 60 days before the date of the meeting. The presence in person or by proxy of the holders of record of a majority of our issued and outstanding shares entitled to vote at such meeting constitutes a quorum for the transaction of business at meetings of the stockholders. Special meetings of the stockholders may be called for any purpose only by the board of directors pursuant to a resolution approved by the affirmative vote of a majority of the directors then in office. Except as may be otherwise provided by applicable law, our certificate of incorporation or our amended and restated by-laws, all elections of directors shall be decided by a plurality, and all other questions shall be decided by a majority, of the votes cast by stockholders entitled to vote thereon at a duly held meeting of stockholders at which a quorum is present.

------

[**Table of Contents**](#TOC)

***Voting Rights***

Holders of common stock are entitled to one vote for each share held of record on all matters to be voted upon by stockholders and do not have cumulative voting rights.

***Dividends***

Subject to the rights, powers and preferences of any outstanding preferred stock that we may designate and issue in the future, and except as provided by law or in our certificate of incorporation, dividends may be declared and paid or set aside for payment on the Common Stock out of legally available assets or funds when and as declared by our board of directors.

***Liquidation, Dissolution and Winding Up***

Subject to the rights, powers and preferences of any outstanding preferred stock that we may designate and issue in the future, in the event of our liquidation, dissolution or winding up, our net assets will be distributed pro rata to the holders of Common Stock.

***Other Rights***

Holders of Common Stock have no preemptive, subscription, redemption or conversion rights. The rights, preferences and privileges of holders of Common Stock are subject to and may be adversely affected by the rights of the holders of shares of any series of preferred stock that we may designate and issue in the future. Holders of Common Stock are not required to make additional capital contributions.

***Transfer Agent***

The transfer agent and registrar for the common stock is Continental Stock Transfer & Trust Company, New York, New York.

**Preferred Stock**

Under our charter, we are currently authorized to issue 1,000,000 shares of preferred stock, par value $.0001 per share. As of December 31, 2024, we had no shares of preferred stock issued and outstanding.

Under our charter, our board of directors is expressly granted authority to issue shares of preferred stock, in one or more series, and to fix for each series such voting powers, full or limited, and such designations, preferences and relative, participating, optional or other special rights and such qualifications, limitations or restrictions as it may determine in the resolution or resolutions providing for the issue of such series (to which we also refer as a "preferred stock designation") and as may be permitted by the Delaware General Corporation Law. The number of authorized shares of preferred stock may be increased or decreased (but not below the number of shares of preferred stock then outstanding) by the affirmative vote of the holders of a majority of the voting power of all of the then outstanding shares of our capital stock entitled to vote generally in the election of directors, voting together as a single class, without a separate vote of the holders of the preferred stock, or any series of preferred stock, unless a vote of any such holders is required pursuant to any preferred stock designation.

The rights and terms relating to any new series of preferred stock could adversely affect the voting power or other rights of the holders of the common stock or could be utilized, under certain circumstances, as a method of discouraging, delaying or preventing a change in control of the Company.

**Provisions of Our Certificate of Incorporation and Amended and Restated By-laws and Delaware Law That May Have Anti-Takeover Effects**

------

[**Table of Contents**](#TOC)

The provisions of Delaware law and our certificate of incorporation and amended and restated by-laws could discourage or make it more difficult to accomplish a proxy contest or other change in our management or the acquisition of control by a holder of a substantial amount of our voting stock. It is possible that these provisions could make it more difficult to accomplish, or could deter, transactions that stockholders may otherwise consider to be in their best interests or in our best interests. These provisions are intended to enhance the likelihood of continuity and stability in the composition of our board of directors and in the policies formulated by the board of directors and to discourage certain types of transactions that may involve an actual or threatened change of our control. These provisions are designed to reduce our vulnerability to an unsolicited acquisition proposal and to discourage certain tactics that may be used in proxy fights. Such provisions also may have the effect of preventing changes in our management.

***Removal of Directors by Stockholders***

Our amended and restated bylaws provide that members of our board of directors may only be removed for cause by a vote of the holders of a majority of the voting power of the outstanding shares entitled to vote on the election of the directors, voting together as a single class.

***Issuance of Preferred Stock***

Our board of directors is authorized, without further action by our stockholders, to issue up to 1,000,000 shares of preferred stock in one or more series, and to fix the designations, powers, preferences and the relative, participating, optional or other special rights, and any qualifications, limitations and restrictions of the shares of each series of preferred stock. The issuance of preferred stock could impede the completion of a merger, tender offer or other takeover attempt.

***Stockholder Nomination of Directors***

Our amended and restated bylaws provide that a stockholder must notify us in writing of any stockholder nomination of a director not earlier than 5:00 p.m., Eastern Time, on the 120th day and not later than 5:00 p.m., Eastern Time, on the 90th day prior to the first anniversary of the preceding year's annual meeting; provided, that if there was no annual meeting in the prior year or if the date of the current year's annual meeting is more than 30 days before or after the anniversary date of the prior year's annual meeting, notice by the stockholder to be timely must be so delivered on or before 10 days after the day on which the date of the current year's annual meeting is first disclosed in a public announcement by us.

***No Action By Written Consent***

Our certificate of incorporation provides that our stockholders may not act by written consent and may only act at duly called meetings of stockholders.

***Delaware Business Combination Statute***

Section 203 of the General Corporation Law of the State of Delaware, which we refer to as the DGCL, is applicable to us. Section 203 of the DGCL restricts some types of transactions and business combinations between a corporation and a 15% stockholder. A 15% stockholder is generally considered by Section 203 to be a person owning 15% or more of the corporation's outstanding voting stock. Section 203 refers to a 15% stockholder as an "interested stockholder." Section 203 restricts these transactions for a period of three years from the date the stockholder acquires 15% or more of our outstanding voting stock. With some exceptions, unless the transaction is approved by the board of directors and the holders of at least two-thirds of the outstanding voting stock of the corporation, Section 203 prohibits significant business transactions such as:

● a merger with, disposition of significant assets to or receipt of disproportionate financial benefits by the interested stockholder, and

● any other transaction that would increase the interested stockholder's proportionate ownership of any class or series of our capital stock.

------

[**Table of Contents**](#TOC)

The shares held by the interested stockholder are not counted as outstanding when calculating the two-thirds of the outstanding voting stock needed for approval.

The prohibition does not apply if:

● prior to the time that any stockholder became an interested stockholder, the board of directors approved either the business combination or the transaction in which such stockholder acquired 15% or more of our outstanding voting stock, or

● the interested stockholder owns at least 75% of our outstanding voting stock as a result of a transaction in which such stockholder acquired 15% or more of our outstanding voting stock. Shares held by persons who are both directors and officers or by some types of employee stock plans are not counted as outstanding when making this calculation.

***Choice of forum***

------

[**Table of Contents**](#TOC)

#### DESCRIPTION OF DEBT SECURITIES
We may offer debt securities that may be senior or subordinated. We refer to senior debt securities and subordinated debt securities collectively as debt securities. Each series of debt securities may have different terms. The following description summarizes the general terms and provisions of the debt securities we might issue. We will describe the specific terms of the debt securities we offer for sale and the extent, if any, to which the general provisions summarized below apply to any series of debt securities in the prospectus supplement relating to the series and any applicable free writing prospectus that we authorize to be delivered.

We may issue senior debt securities from time to time, in one or more series under a senior indenture to be entered into between us and a senior trustee to be named in a prospectus supplement, which we refer to as the senior trustee. We may issue subordinated debt securities from time to time, in one or more series, under a subordinated indenture to be entered into between us and a subordinated trustee to be named in a prospectus supplement, which we refer to as the subordinated trustee. The forms of senior indenture and subordinated indenture are filed as exhibits to the registration statement of which this prospectus forms a part. Together, the senior indenture and the subordinated indenture are referred to as the indentures and, together, the senior trustee and the subordinated trustee are referred to as the trustees. This prospectus briefly outlines some of the provisions of the indentures. The following summary of the material provisions of the indentures is qualified in its entirety by the provisions of the indentures, including definitions of certain terms used in the indentures. Wherever we refer to particular sections or defined terms of the indentures, those sections or defined terms are incorporated by reference in this prospectus or the applicable prospectus supplement. You should review the indentures that are filed as exhibits to the registration statement of which this prospectus forms a part for additional information. As used in this prospectus, the term "debt securities" includes the debt securities being offered by this prospectus and all other debt securities issued by us under the indentures.

#### General
The indentures:

● do not limit the amount of debt securities that we may issue;

● allow us to issue debt securities in one or more series;

● do not require us to issue all of the debt securities of a series at the same time; and

● allow us to reopen a series to issue additional debt securities without the consent of the holders of the debt securities of such series.

Unless otherwise provided in the applicable prospectus supplement, the senior debt securities will be unsubordinated obligations and will rank equally with all of our other unsecured and unsubordinated indebtedness. Payments on the subordinated debt securities will be subordinated to the prior payment in full of all of our senior indebtedness, as described under "Subordination" and in the applicable prospectus supplement.

Each indenture provides that we may, but need not, designate more than one trustee under an indenture. Any trustee under an indenture may resign or be removed and a successor trustee may be appointed to act with respect to the series of debt securities administered by the resigning or removed trustee. If two or more persons are acting as trustee with respect to different series of debt securities, each trustee shall be a trustee of a trust under the applicable indenture separate and apart from the trust administered by any other trustee. Except as otherwise indicated in this prospectus, any action described in this prospectus to be taken by each trustee may be taken by each trustee with respect to, and only with respect to, the one or more series of debt securities for which it is trustee under the applicable indenture.

The prospectus supplement for each offering will provide the following terms, where applicable:

● the title of the debt securities and whether they are senior or subordinated;

● any limit upon the aggregate principal amount of the debt securities of that series;

● the date or dates on which the principal of the debt securities of the series is payable;

● the price at which the debt securities will be issued, expressed as a percentage of the principal and, if other than the principal amount thereof, the portion of the principal amount thereof payable upon declaration of acceleration of the maturity thereof or, if applicable, the portion of the principal amount of such debt securities that is convertible into another security of ours or the method by which any such portion shall be determined;

------

[**Table of Contents**](#TOC)

● the rate or rates at which the debt securities of the series shall bear interest or the manner of calculation of such rate or rates, if any;

● the date or dates from which interest will accrue, the interest payment dates on which such interest will be payable or the manner of determination of such interest payment dates, the place(s) of payment, and the record date for the determination of holders to whom interest is payable on any such interest payment dates or the manner of determination of such record dates;

● the right, if any, to extend the interest payment periods and the duration of such extension;

● the period or periods within which, the price or prices at which and the terms and conditions upon which debt securities of the series may be redeemed, converted or exchanged, in whole or in part;

● our obligation, if any, to redeem or purchase debt securities of the series pursuant to any sinking fund, mandatory redemption, or analogous provisions (including payments made in cash in satisfaction of future sinking fund obligations) or at the option of a holder thereof and the period or periods within which, the price or prices at which, and the terms and conditions upon which, debt securities of the series shall be redeemed or purchased, in whole or in part, pursuant to such obligation;

● the form of the debt securities of the series including the form of the Certificate of Authentication for such series;

● if other than minimum denominations of one thousand U.S. dollars ($1,000) or any integral multiple of $1,000 thereof, the denominations in which the debt securities of the series shall be issuable;

● whether the debt securities of the series shall be issued in whole or in part in the form of a global debt security or global debt securities; the terms and conditions, if any, upon which such global debt security or global debt securities may be exchanged in whole or in part for other individual debt securities; and the depositary for such global debt security or global debt securities;

● whether the debt securities will be convertible into or exchangeable for common stock or other securities of ours or any other Person and, if so, the terms and conditions upon which such debt securities will be so convertible or exchangeable, including the conversion or exchange price, as applicable, or how it will be calculated and may be adjusted, any mandatory or optional (at our option or the holders' option) conversion or exchange features, and the applicable conversion or exchange period;

● any additional or alternative events of default to those set forth in the indenture;

● any additional or alternative covenants to those set forth in the indenture;

● the currency or currencies including composite currencies, in which payment of the principal of (and premium, if any) and interest, if any, on such debt securities shall be payable (if other than the currency of the United States of America), which unless otherwise specified shall be the currency of the United States of America as at the time of payment is legal tender for payment of public or private debts;

● if the principal of (and premium, if any), or interest, if any, on such debt securities is to be payable, at our election or at the election of any holder thereof, in a coin or currency other than that in which such debt securities are stated to be payable, then the period or periods within which, and the terms and conditions upon which, such election may be made;

● whether interest will be payable in cash or additional debt securities at our or the holders' option and the terms and conditions upon which the election may be made;

● the terms and conditions, if any, upon which we will pay amounts in addition to the stated interest, premium, if any and principal amounts of the debt securities of the series to any holder that is not a "United States person" for federal tax purposes;

● additional or alternative provisions, if any, related to defeasance and discharge of the offered debt securities than those set forth in the indenture;

● the applicability of any guarantees;

● any restrictions on transfer, sale or assignment of the debt securities of the series; and

● any other terms of the debt securities (which may supplement, modify or delete any provision of the indenture insofar as it applies to such series).

------

[**Table of Contents**](#TOC)

We may issue debt securities that provide for less than the entire principal amount thereof to be payable upon declaration of acceleration of the maturity of the debt securities. We refer to any such debt securities throughout this prospectus as "original issue discount securities."

We will provide you with more information in the applicable prospectus supplement regarding any deletions, modifications, or additions to the events of default or covenants that are described below, including any addition of a covenant or other provision providing event risk or similar protection.

#### Payment
Unless otherwise provided in the applicable prospectus supplement, the principal of, and any premium or make-whole amount, and interest on, any series of the debt securities will be payable by mailing a check to the address of the person entitled to it as it appears in the applicable register for the debt securities or by wire transfer of funds to that person at an account maintained within the United States.

All monies that we pay to a paying agent or a trustee for the payment of the principal of, and any premium, or interest on, any debt security will be repaid to us if unclaimed at the end of two years after the obligation underlying payment becomes due and payable. After funds have been returned to us, the holder of the debt security may look only to us for payment, without payment of interest for the period which we hold the funds.

#### Merger, Consolidation or Sale of Assets
The indentures provide that we may, without the consent of the holders of any outstanding debt securities, (i) consolidate with, (ii) sell, lease or convey all or substantially all of our assets to, or (iii) merge with or into, any other entity, provided that:

● either we are the continuing entity, or the successor entity, if other than us, assumes the obligations (a) to pay the principal of, and any premium, and interest on, all of the debt securities and (b) to duly perform and observe all of the covenants and conditions contained in the applicable indenture; and in the event the debt securities are convertible into or exchangeable for common stock or other securities of ours, such successor entity will, by such supplemental indenture, make provision so that the holders of debt securities of that series shall thereafter be entitled to receive upon conversion or exchange of such debt securities the number of securities or property to which a holder of the number of common stock or other securities of ours deliverable upon conversion or exchange of those debt securities would have been entitled had such conversion or exchange occurred immediately prior to such consolidation, merger, sale, conveyance, transfer or other disposition; and

● an officers' certificate and legal opinion covering such conditions are delivered to each applicable trustee.

#### Events of Default, Notice and Waiver
Unless the applicable prospectus supplement states otherwise, when we refer to "events of default" as defined in the indentures with respect to any series of debt securities, we mean:

● default in the payment of any installment of interest on any debt security of such series continuing for 90 days unless such date has been extended or deferred;

● default in the payment of principal of, or any premium on, any debt security of such series when due and payable unless such date has been extended or deferred;

● default in the performance or breach of any covenant or warranty in the debt securities or in the indenture by us continuing for 90 days after written notice described below;

● bankruptcy insolvency or reorganization, or court appointment of a receiver, liquidator or trustee of us; and

● any other event of default provided with respect to a particular series of debt securities.

If an event of default (other than an event of default described in the fourth bullet point above) occurs and is continuing with respect to debt securities of any series outstanding, then the applicable trustee or the holders of 25% or more in principal amount of the debt securities of that series will have the right to declare the principal amount of, and accrued interest on, all the debt securities of that series to be due and payable. If an event of default described in the fourth bullet point above occurs, the principal amount of, and accrued interest on, all the debt securities of that series will

------

[**Table of Contents**](#TOC)

automatically become and will be immediately due and payable without any declaration or other act on the part of the trustee or the holders of the debt securities. However, at any time after such a declaration of acceleration has been made, but before a judgment or decree for payment of the money due has been obtained by the applicable trustee, the holders of at least a majority in principal amount of outstanding debt securities of such series or of all debt securities then outstanding under the applicable indenture may rescind and annul such declaration and its consequences if:

● we have deposited with the applicable trustee all required payments of the principal, any premium, interest and, to the extent permitted by law, interest on overdue installment of interest, plus applicable fees, expenses, disbursements and advances of the applicable trustee; and

● all events of default, other than the non-payment of accelerated principal, or a specified portion thereof, and any premium, have been cured or waived.

The indentures provide that holders of debt securities of any series may not institute any proceedings, judicial or otherwise, with respect to such indenture or for any remedy under the indenture, unless the trustee fails to act for a period of 90 days after the trustee has received a written request to institute proceedings in respect of an event of default from the holders of 25% or more in principal amount of the outstanding debt securities of such series, as well as an offer of indemnity reasonably satisfactory to the trustee. However, this provision will not prevent any holder of debt securities from instituting suit for the enforcement of payment of the principal of, and any premium, and interest on, such debt securities at the respective due dates thereof.

The indentures provide that, subject to provisions in each indenture relating to its duties in the case of a default, a trustee has no obligation to exercise any of its rights or powers at the request or direction of any holders of any series of debt securities then outstanding under the indenture, unless the holders have offered to the trustee reasonable security or indemnity. The holders of at least a majority in principal amount of the outstanding debt securities of any series or of all debt securities then outstanding under an indenture shall have the right to direct the time, method and place of conducting any proceeding for any remedy available to the applicable trustee, or of exercising any trust or power conferred upon such trustee. However, a trustee may refuse to follow any direction which:

● is in conflict with any law or the applicable indenture;

● may involve the trustee in personal liability; or

● may be unduly prejudicial to the holders of debt securities of the series not joining the proceeding.

Within 120 days after the close of each fiscal year, we will be required to deliver to each trustee a certificate, signed by one of our several specified officers, stating whether or not that officer has knowledge of any default under the applicable indenture. If the officer has knowledge of any default, the notice must specify the nature and status of the default.

#### Modification of the Indentures
Subject to certain exceptions, the indentures may be amended with the consent of the holders of a majority in aggregate principal amount of the outstanding debt securities of all series affected by such amendment (including consents obtained in connection with a tender offer or exchange for the debt securities of such series).

We and the applicable trustee may make modifications and amendments of an indenture without the consent of any holder of debt securities for any of the following purposes:

● to cure any ambiguity, defect, or inconsistency in the applicable indenture or in the Securities of any series;

● to comply with the covenant described above under "Merger, Consolidation or Sale of Assets;"

● to provide for uncertificated debt securities in addition to or in place of certificated debt securities;

● to add events of default for the benefit of the holders of all or any series of debt securities;

● to add the covenants, restrictions, conditions or provisions relating to us for the benefit of the holders of all or any series of debt securities (and if such covenants, restrictions, conditions or provisions are to be for the benefit of less than all series of debt securities, stating that such covenants, restrictions, conditions or provisions are expressly being included solely for the benefit of such series), to make the occurrence, or the occurrence and the continuance, of a default in any such additional covenants, restrictions, conditions or provisions an event of default, or to surrender any right or power in the applicable indenture conferred upon us;

------

[**Table of Contents**](#TOC)

● to add to, delete from, or revise the conditions, limitations, and restrictions on the authorized amount, terms, or purposes of issue, authentication, and delivery of debt securities, as set forth in the applicable indenture;

● to make any change that does not adversely affect the rights of any holder of notes under the applicable indenture in any material respect;

● to provide for the issuance of and establish the form and terms and conditions of the debt securities of any series as provided in the applicable indenture, to establish the form of any certifications required to be furnished pursuant to the terms of the applicable indenture or any series of debt securities under the applicable indenture, or to add to the rights of the holders of any series of debt securities;

● to evidence and provide for the acceptance of appointment under the applicable indenture by a successor trustee or to appoint a separate trustee with respect to any series;

● to comply with any requirements of the SEC or any successor in connection with the qualification of the indenture under the Trust Indenture Act of 1939, as amended, or the Trust Indenture Act; or

● to conform the applicable indenture to this "Description of Debt Securities" or any other similarly titled section in any prospectus supplement or other offering document relating to a series of debt securities.

#### Subordination
Payment by us of the principal of, premium, if any, and interest on any series of subordinated debt securities issued under the subordinated indenture will be subordinated to the extent set forth in an indenture supplemental to the subordinated indenture relating to such series.

#### Discharge, Defeasance and Covenant Defeasance
Unless otherwise provided in the applicable prospectus supplement, the indentures allow us to discharge our obligations to holders of any series of debt securities issued under any indenture when:

● either (i) all securities of such series have already been delivered to the applicable trustee for cancellation; or (ii) all securities of such series have not already been delivered to the applicable trustee for cancellation but (a) have become due and payable, (b) will become due and payable within one year, or (c) if redeemable at our option, are to be redeemed within one year, and we have irrevocably deposited with the applicable trustee, in trust, funds in such currency or currencies, or governmental obligations in an amount sufficient to pay the entire indebtedness on such debt securities in respect of principal and any premium, and interest to the date of such deposit if such debt securities have become due and payable or, if they have not, to the stated maturity or redemption date; and

● we have paid or caused to be paid all other sums payable.

Unless otherwise provided in the applicable prospectus supplement, the indentures provide that, upon our irrevocable deposit with the applicable trustee, in trust, of an amount, in such currency or currencies in which such debt securities are payable at stated maturity, or government obligations, or both, applicable to such debt securities, which through the scheduled payment of principal and interest in accordance with their terms will provide money in an amount sufficient to pay the principal of, and any premium or make-whole amount, and interest on, such debt securities, and any mandatory sinking fund or analogous payments thereon, on the scheduled due dates therefor, the issuing company shall be released from its obligations with respect to such debt securities under the applicable indenture or, if provided in the applicable prospectus supplement, its obligations with respect to any other covenant, and any omission to comply with such obligations shall not constitute an event of default with respect to such debt securities.

The applicable prospectus supplement may further describe the provisions, if any, permitting such defeasance or covenant defeasance, including any modifications to the provisions described above, with respect to the debt securities of or within a particular series.

#### Conversion Rights
The terms and conditions, if any, upon which the debt securities are convertible into common stock or other securities of ours will be set forth in the applicable prospectus supplement. The terms will include whether the debt securities are convertible into shares of common stock or other securities of ours, the conversion price, or manner of calculation

------

[**Table of Contents**](#TOC)

thereof, the conversion period, provisions as to whether conversion will be at the issuing company's option or the option of the holders, the events requiring an adjustment of the conversion price and provisions affecting conversion in the event of the redemption of the debt securities and any restrictions on conversion.

#### Governing Law
The indentures and the debt securities will be governed by and construed in accordance with the laws of the State of New York, except to the extent that the Trust Indenture Act is applicable.

------

[**Table of Contents**](#TOC)

#### DESCRIPTION OF WARRANTS
The following description, together with the additional information we may include in any applicable prospectus supplements, summarizes the material terms and provisions of the warrants that we may offer under this prospectus and the related warrant agreements and warrant certificates. While the terms summarized below will apply generally to any warrants that we may offer, we will describe the particular terms of any series of warrants in more detail in the applicable prospectus supplement. If we indicate in the applicable prospectus supplement, the terms of any warrants offered under that prospectus supplement may differ from the terms described below. Specific warrant agreements will contain additional important terms and provisions and will be incorporated by reference as an exhibit to the registration statement, which includes this prospectus.

#### General
We may issue warrants for the purchase of common stock, preferred stock and/or debt securities in one or more series. We may issue warrants independently or together with common stock, preferred stock and/or debt securities, and the warrants may be attached to or separate from these securities.

We will evidence each series of warrants by warrant certificates that we will issue under a separate warrant agreement. We will enter into the warrant agreement with a warrant agent. We will indicate the name and address of the warrant agent in the applicable prospectus supplement relating to a particular series of warrants.

We will describe in the applicable prospectus supplement the terms of the series of warrants, including:

● the offering price and aggregate number of warrants offered;

● the currency for which the warrants may be purchased;

● if applicable, the designation and terms of the securities with which the warrants are issued and the number of warrants issued with each such security or each principal amount of such security;

● if applicable, the date on and after which the warrants and the related securities will be separately transferable;

● in the case of warrants to purchase debt securities, the principal amount of debt securities purchasable upon exercise of one warrant and the price at, and currency in which, this principal amount of debt securities may be purchased upon such exercise;

● in the case of warrants to purchase common stock or preferred stock, the number of shares of common stock or preferred stock, as the case may be, purchasable upon the exercise of one warrant and the price at which these shares may be purchased upon such exercise;

● the effect of any merger, consolidation, sale or other disposition of our business on the warrant agreement and the warrants;

● the terms of any rights to redeem or call the warrants;

● any provisions for changes to or adjustments in the exercise price or number of securities issuable upon exercise of the warrants;

● the periods during which, and places at which, the warrants are exercisable;

● the manner of exercise;

● the dates on which the right to exercise the warrants will commence and expire;

● the manner in which the warrant agreement and warrants may be modified;

● federal income tax consequences of holding or exercising the warrants;

● the terms of the securities issuable upon exercise of the warrants; and

● any other specific terms, preferences, rights or limitations of or restrictions on the warrants .

------

[**Table of Contents**](#TOC)

#### DESCRIPTION OF UNITS
We may issue units comprised of shares of common stock, shares of preferred stock, debt securities and warrants in any combination. We may issue units in such amounts and in as many distinct series as we wish. This section outlines certain provisions of the units that we may issue. If we issue units, they will be issued under one or more unit agreements to be entered into between us and a bank or other financial institution, as unit agent. The information described in this section may not be complete in all respects and is qualified entirely by reference to the unit agreement with respect to the units of any particular series. The specific terms of any series of units offered will be described in the applicable prospectus supplement. If so described in a particular prospectus supplement, the specific terms of any series of units may differ from the general description of terms presented below. We urge you to read any prospectus supplement related to any series of units we may offer, as well as the complete unit agreement and unit certificate that contain the terms of the units. If we issue units, forms of unit agreements and unit certificates relating to such units will be incorporated by reference as exhibits to the registration statement, which includes this prospectus.

Each unit that we may issue will be issued so that the holder of the unit is also the holder of each security included in the unit. Thus, the holder of a unit will have the rights and obligations of a holder of each included security. The unit agreement under which a unit is issued may provide that the securities included in the unit may not be held or transferred separately, at any time or at any time before a specified date. The applicable prospectus supplement may describe:

● the designation and terms of the units and of the securities comprising the units, including whether and under what circumstances those securities may be held or transferred separately;

● any provisions of the governing unit agreement;

● the price or prices at which such units will be issued;

● the applicable United States federal income tax considerations relating to the units;

● any provisions for the issuance, payment, settlement, transfer or exchange of the units or of the securities comprising the units; and

● any other terms of the units and of the securities comprising the units.

The provisions described in this section, as well as those described under "Description of Capital Stock," "Description of Debt Securities" and "Description of Warrants" will apply to the securities included in each unit, to the extent relevant and as may be updated in any prospectus supplements.

#### Issuance in Series
We may issue units in such amounts and in as many distinct series as we wish. This section summarizes terms of the units that apply generally to all series. Most of the financial and other specific terms of a particular series of units will be described in the applicable prospectus supplement.

#### Unit Agreements
We will issue the units under one or more unit agreements to be entered into between us and a bank or other financial institution, as unit agent. We may add, replace or terminate unit agents from time to time. We will identify the unit agreement under which each series of units will be issued and the unit agent under that agreement in the applicable prospectus supplement.

The following provisions will generally apply to all unit agreements unless otherwise stated in the applicable prospectus supplement:

#### Modification without Consent
We and the applicable unit agent may amend any unit or unit agreement without the consent of any holder:

● to cure any ambiguity, including modifying any provisions of the governing unit agreement that differ from those described below;

● to correct or supplement any defective or inconsistent provision; or

------

[**Table of Contents**](#TOC)

● to make any other change that we believe is necessary or desirable and will not adversely affect the interests of the affected holders in any material respect.

We do not need any approval to make changes that affect only units to be issued after the changes take effect. We may also make changes that do not adversely affect a particular unit in any material respect, even if they adversely affect other units in a material respect. In those cases, we do not need to obtain the approval of the holder of the unaffected unit; we need only obtain any required approvals from the holders of the affected units.

#### Modification with Consent
We may not amend any particular unit or a unit agreement with respect to any particular unit unless we obtain the consent of the holder of that unit, if the amendment would:

● impair any right of the holder to exercise or enforce any right under a security included in the unit if the terms of that security require the consent of the holder to any changes that would impair the exercise or enforcement of that right; or

● reduce the percentage of outstanding units or any series or class the consent of whose holders is required to amend that series or class, or the applicable unit agreement with respect to that series or class, as described below.

Any other change to a particular unit agreement and the units issued under that agreement would require the following approval:

● if the change affects only the units of a particular series issued under that agreement, the change must be approved by the holders of a majority of the outstanding units of that series; or

● if the change affects the units of more than one series issued under that agreement, it must be approved by the holders of a majority of all outstanding units of all series affected by the change, with the units of all the affected series voting together as one class for this purpose.

These provisions regarding changes with majority approval also apply to changes affecting any securities issued under a unit agreement, as the governing document.

In each case, the required approval must be given by written consent.

#### Unit Agreements will not be Qualified Under Trust Indenture Act
No unit agreement will be qualified as an indenture, and no unit agent will be required to qualify as a trustee, under the Trust Indenture Act. Therefore, holders of units issued under unit agreements will not have the protections of the Trust Indenture Act with respect to their units.

#### Mergers and Similar Transactions Permitted; No Restrictive Covenants or Events of Default
The unit agreements will not restrict our ability to merge or consolidate with, or sell our assets to, another corporation or other entity or to engage in any other transactions. If at any time we merge or consolidate with, or sell our assets substantially as an entirety to, another corporation or other entity, the successor entity will succeed to and assume our obligations under the unit agreements. We will then be relieved of any further obligation under these agreements.

The unit agreements will not include any restrictions on our ability to put liens on our assets, nor will they restrict our ability to sell our assets. The unit agreements also will not provide for any events of default or remedies upon the occurrence of any events of default.

#### Governing Law
The unit agreements and the units will be governed by Delaware law.

------

[**Table of Contents**](#TOC)

#### Form, Exchange and Transfer
We will issue each unit in global—i.e., book-entry—form only. Units in book-entry form will be represented by a global security registered in the name of a depositary, which will be the holder of all the units represented by the global security. Those who own beneficial interests in a unit will do so through participants in the depositary's system, and the rights of these indirect owners will be governed solely by the applicable procedures of the depositary and its participants. We will describe book-entry securities, and other terms regarding the issuance and registration of the units in the applicable prospectus supplement.

Each unit and all securities comprising the unit will be issued in the same form.

If we issue any units in registered, non-global form, the following will apply to them. The units will be issued in the denominations stated in the applicable prospectus supplement. Holders may exchange their units for units of smaller denominations or combined into fewer units of larger denominations, as long as the total amount is not changed.

● holders may exchange or transfer their units at the office of the unit agent. Holders may also replace lost, stolen, destroyed or mutilated units at that office. We may appoint another entity to perform these functions or perform them ourselves.

● holders will not be required to pay a service charge to transfer or exchange their units, but they may be required to pay for any tax or other governmental charge associated with the transfer or exchange. The transfer or exchange, and any replacement, will be made only if our transfer agent is satisfied with the holder's proof of legal ownership. The transfer agent may also require an indemnity before replacing any units.

● if we have the right to redeem, accelerate or settle any units before their maturity, and we exercise our right as to less than all those units or other securities, we may block the exchange or transfer of those units during the period beginning 15 days before the day we mail the notice of exercise and ending on the day of that mailing, in order to freeze the list of holders to prepare the mailing. We may also refuse to register transfers of or exchange any unit selected for early settlement, except that we will continue to permit transfers and exchanges of the unsettled portion of any unit being partially settled. We may also block the transfer or exchange of any unit in this manner if the unit includes securities that are or may be selected for early settlement.

Only the depositary will be entitled to transfer or exchange a unit in global form, since it will be the sole holder of the unit.

#### Payments and Notices
In making payments and giving notices with respect to our units, we will follow the procedures as described in the applicable prospectus supplement.

------

[**Table of Contents**](#TOC)

#### PLAN OF DISTRIBUTION
We may sell securities:

● through underwriters;

● through dealers;

● through agents;

● directly to purchasers; or

● through a combination of any of these methods or any other method permitted by law.

In addition, we may issue the securities as a dividend or distribution or in a subscription rights offering to our existing security holders.

We may directly solicit offers to purchase securities, or we may designate agents to solicit these offers. In the prospectus supplement relating to such an offering, we will name any agent that could be viewed as an underwriter under the Securities Act and describe any commissions that we must pay to any such agent. Any such agent will be acting on a best efforts basis for the period of its appointment or, if indicated in the applicable prospectus supplement, on a firm commitment basis. This prospectus may be used in connection with any offering of our securities through any of these methods or other methods described in the applicable prospectus supplement.

The distribution of the securities may be effected from time to time in one or more transactions:

● at a fixed price, or prices, which may be changed from time to time;

● at market prices prevailing at the time of sale;

● at prices related to such prevailing market prices; or

● at negotiated prices.

Each prospectus supplement will describe the method of distribution of the securities and any applicable restrictions.

The prospectus supplement with respect to the securities of a particular series will describe the terms of the offering of the securities, including the following:

● the name of the agent or any underwriters;

● the public offering or purchase price;

● any discounts and commissions to be allowed or paid to the agent or underwriters;

● all other items constituting underwriting compensation;

● any discounts and commissions to be allowed or paid to dealers; and

● any exchanges on which the securities will be listed.

If any underwriters or agents are used in the sale of the securities in respect of which this prospectus is delivered, we will enter into an underwriting agreement, equity distribution agreement or other agreement with them at the time of sale to them, and we will set forth in the prospectus supplement relating to such offering the names of the underwriters or agents and the terms of the related agreement with them.

In connection with the offering of securities, we may grant to the underwriters an option to purchase additional securities with an additional underwriting commission, as may be set forth in the accompanying prospectus supplement. If we grant any such option, the terms of such option will be set forth in the prospectus supplement for such securities.

If a dealer is used in the sale of the securities in respect of which the prospectus is delivered, we will sell such securities to the dealer, as principal. The dealer, who may be deemed to be an "underwriter" as that term is defined in the Securities Act, may then resell such securities to the public at varying prices to be determined by such dealer at the time of resale.

If we offer securities in a subscription rights offering to our existing security holders, we may enter into a standby underwriting agreement with dealers, acting as standby underwriters. We may pay the standby underwriters a commitment fee for the securities they commit to purchase on a standby basis. If we do not enter into a standby

------

[**Table of Contents**](#TOC)

underwriting arrangement, we may retain a dealer-manager to manage a subscription rights offering for us. Agents, underwriters, dealers and other persons may be entitled under agreements which they may enter into with us to indemnification by us against certain civil liabilities, including liabilities under the Securities Act, and may be customers of, engage in transactions with or perform services for us in the ordinary course of business.

If so indicated in the applicable prospectus supplement, we will authorize underwriters or other persons acting as our agents to solicit offers by certain institutions to purchase securities from us pursuant to delayed delivery contracts providing for payment and delivery on the date stated in the prospectus supplement. Each contract will be for an amount not less than, and the aggregate amount of securities sold pursuant to such contracts shall not be less nor more than, the respective amounts stated in the prospectus supplement. Institutions with whom the contracts, when authorized, may be made include commercial and savings banks, insurance companies, pension funds, investment companies, educational and charitable institutions and other institutions, but shall in all cases be subject to our approval. Delayed delivery contracts will not be subject to any conditions except that:

● the purchase by an institution of the securities covered under that contract shall not at the time of delivery be prohibited under the laws of the jurisdiction to which that institution is subject; and

● if the securities are also being sold to underwriters acting as principals for their own account, the underwriters shall have purchased such securities not sold for delayed delivery. The underwriters and other persons acting as our agents will not have any responsibility in respect of the validity or performance of delayed delivery contracts.

Offered securities may also be offered and sold, if so indicated in the prospectus supplement, in connection with a remarketing upon their purchase, in accordance with a redemption or repayment pursuant to their terms, or otherwise, by one or more remarketing firms, acting as principals for their own accounts or as agents for us. Any remarketing firm will be identified and the terms of its agreement, if any, with us and its compensation will be described in the applicable prospectus supplement. Remarketing firms may be deemed to be underwriters in connection with their remarketing of offered securities.

Certain agents, underwriters and dealers, and their associates and affiliates, may be customers of, have borrowing relationships with, engage in other transactions with, or perform services, including investment banking services, for us or one or more of our respective affiliates in the ordinary course of business.

In order to facilitate the offering of the securities, any underwriters may engage in transactions that stabilize, maintain or otherwise affect the price of the securities or any other securities the prices of which may be used to determine payments on such securities. Specifically, any underwriters may over-allot in connection with the offering, creating a short position for their own accounts. In addition, to cover overallotments or to stabilize the price of the securities or of any such other securities, the underwriters may bid for, and purchase, the securities or any such other securities in the open market. Finally, in any offering of the securities through a syndicate of underwriters, the underwriting syndicate may reclaim selling concessions allowed to an underwriter or a dealer for distributing the securities in the offering if the syndicate repurchases previously distributed securities in transactions to cover syndicate short positions, in stabilization transactions or otherwise. Any of these activities may stabilize or maintain the market price of the securities above independent market levels. Any such underwriters are not required to engage in these activities and may end any of these activities at any time.

We may engage in at the market offerings into an existing trading market in accordance with Rule 415(a)(4) under the Securities Act. In addition, we may enter into derivative transactions with third parties, or sell securities not covered by this prospectus to third parties in privately negotiated transactions. If the applicable prospectus supplement so indicates, in connection with those derivatives, the third parties may sell securities covered by this prospectus and the applicable prospectus supplement, including in short sale transactions. If so, the third party may use securities pledged by us or borrowed from us or others to settle those sales or to close out any related open borrowings of stock, and may use securities received from us in settlement of those derivatives to close out any related open borrowings of stock. The third party in such sale transactions will be an underwriter and, if not identified in this prospectus, will be named in the applicable prospectus supplement (or a post-effective amendment). In addition, we may otherwise loan or pledge securities to a financial institution or other third party that in turn may sell the securities short using this prospectus and

------

[**Table of Contents**](#TOC)

an applicable prospectus supplement. Such financial institution or other third party may transfer its economic short position to investors in our securities or in connection with a concurrent offering of other securities.

Under Rule 15c6-1 of the Exchange Act, trades in the secondary market generally are required to settle in one business day, unless the parties to any such trade expressly agree otherwise. The applicable prospectus supplement may provide that the original issue date for your securities may be more than one scheduled business day after the trade date for your securities. Accordingly, in such a case, if you wish to trade securities on any date prior to the first business day before the original issue date for your securities, you will be required, by virtue of the fact that your securities initially are expected to settle in more than one scheduled business day after the trade date for your securities, to make alternative settlement arrangements to prevent a failed settlement. The securities may be new issues of securities and may have no established trading market. The securities may or may not be listed on a national securities exchange. We can make no assurance as to the liquidity of or the existence of trading markets for any of the securities.

The specific terms of any lock-up provisions in respect of any given offering will be described in the applicable prospectus supplement.

The underwriters, dealers and agents may engage in transactions with us, or perform services for us, in the ordinary course of business for which they receive compensation.

The anticipated date of delivery of offered securities will be set forth in the applicable prospectus supplement relating to each offer.

------

[**Table of Contents**](#TOC)

**LEGAL MATTERS**

Certain legal matters in connection with this offering will be passed upon for us by Goodwin Procter LLP, Boston, Massachusetts. Any underwriters will also be advised about the validity of the securities and other legal matters by their own counsel, which will be named in the prospectus supplement.

**EXPERTS**

The consolidated financial statements of Altimmune, Inc. appearing in Altimmune, Inc.'s Annual Report (Form 10-K) for the year ended December 31, 2024 have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their report thereon, included therein, and incorporated herein by reference. Such consolidated financial statements are incorporated herein by reference in reliance upon such report given on the authority of such firm as experts in accounting and auditing.

------

[**Table of Contents**](#TOC)

**WHERE YOU CAN FIND MORE INFORMATION**

This prospectus is part of a registration statement we filed with the SEC. This prospectus does not contain all of the information set forth in the registration statement and the exhibits to the registration statement. For further information with respect to us and the securities we are offering under this prospectus, we refer you to the registration statement and the exhibits and schedules filed as a part of the registration statement. You should rely only on the information contained in this prospectus or incorporated by reference. We have not authorized anyone else to provide you with different information. We are not making an offer of these securities in any state where the offer is not permitted. You should not assume that the information in this prospectus is accurate as of any date other than the date on the front page of this prospectus, regardless of the time of delivery of this prospectus or any sale of the securities offered by this prospectus.

We file annual, quarterly and current reports, proxy statements, and other information with the SEC. The SEC maintains a web site at http://www.sec.gov that contains reports, proxy statements and other information about issuers, such as us, who file electronically with the SEC. The address of the SEC website is www.sec.gov.

We maintain a website at http://www.altimmune.com. The information contained on, or accessible through, our website is not incorporated by reference into this prospectus and, therefore is not part of this prospectus or part of the accompanying prospectus supplement.

------

[**Table of Contents**](#TOC)

**INCORPORATION BY REFERENCE**

The SEC allows us to incorporate by reference much of the information we file with the SEC, which means that we can disclose important information to you by referring you to those publicly available documents. The information that we incorporate by reference in this prospectus is considered to be part of this prospectus.

Because we are incorporating by reference future filings with the SEC, this prospectus is continually updated and those future filings may modify or supersede some of the information included or incorporated in this prospectus. This means that you must look at all of the SEC filings that we incorporate by reference to determine if any of the statements in this prospectus or in any document previously incorporated by reference have been modified or superseded. This prospectus incorporates by reference the documents listed below and any future filings we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act (in each case, other than those documents or the portions of those documents not deemed to be filed) between the date of the initial registration statement and the effectiveness of the registration statement and following the effectiveness of the registration statement until the offering of the securities under the registration statement is terminated or completed:

● our Annual Report on Form 10-K for the fiscal year ended December 31, 2024, filed with the SEC on February 27, 2025 (the " [2024 10-K](https://www.sec.gov/Archives/edgar/data/1326190/000132619025000015/alt-20241231x10k.htm) ");

● our Current Reports on Form 8-K, as filed with the SEC on [February 25, 2025](https://www.sec.gov/Archives/edgar/data/1326190/000132619025000003/alt-20250224x8k.htm) and [February 27, 2025](https://www.sec.gov/Archives/edgar/data/1326190/000132619025000011/alt-20250224x8k.htm) ; and

● the description of our common stock set forth in the registration statement on Form 8-A registering our common stock under Section 12 of the Exchange Act, which was filed as [Exhibit 4.1](https://www.sec.gov/Archives/edgar/data/1326190/000132619025000015/alt-20241231xex4d1.htm) to our 2024 10-K, including any amendments or reports filed for purposes of updating such description.

We also incorporate by reference into this prospectus all documents (other than current reports furnished under Item 2.02 or Item 7.01 of Form 8-K and exhibits filed on such form that are related to such items) that are filed by us with the SEC pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act (i) after the date of the initial filing of the registration statement of which this prospectus forms a part and prior to effectiveness of the registration statement, and (ii) after the date of this prospectus but prior to the termination of the offering. These documents include, without limitation, Annual Reports on Form 10-K, Quarterly Reports on Form 10-Q, Current Reports on Form 8-K, as well as proxy statements.

You may request a copy of these filings, at no cost, by contacting us, either orally or in writing, at the following:

Altimmune, Inc.

910 Clopper Road, Suite 201S

Gaithersburg, Maryland

Attention: Chief Financial Officer

(240) 654-1450

------

[**Table of Contents**](#TOC)

![Graphic](alt-20251106x424b5009.jpg)

**Up to $200,000,000**

**Common Stock**

------

**PROSPECTUS SUPPLEMENT**

------

**Leerink Partners**

November 6, 2025

------