# EDGAR Filing Document

**Accession Number:** 0001103804
**File Stem:** 0000905148-26-002008
**Filing Date:** 2026-5
**Character Count:** 38595
**Document Hash:** b91baad1c90dfc5d29e8d0ebe7442e88
**Contains OCR:** False
**Source Format:** 

## Filing Content

## Filing Summary
**0000905148-26-002008.hdr.sgml**: 20260501

**ACCESSION NUMBER**: 0000905148-26-002008

**CONFORMED SUBMISSION TYPE**: SCHEDULE 13G

**PUBLIC DOCUMENT COUNT**: 2

**FILED AS OF DATE**: 20260501

**DATE AS OF CHANGE**: 20260501

**SUBJECT COMPANY**: 

**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:** SCHEDULE 13G
- **SEC ACT:** 1934 Act
- **SEC FILE NUMBER:** 005-80900
- **FILM NUMBER:** 26933987

**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
**FILED BY**: 

**COMPANY DATA:**
- **COMPANY CONFORMED NAME:** VIKING GLOBAL INVESTORS LP
- **CENTRAL INDEX KEY:** 0001103804

**ORGANIZATION NAME:**
- **EIN:** 134055118
- **STATE OF INCORPORATION:** DE
- **FISCAL YEAR END:** 1231

**FILING VALUES:**
- **FORM TYPE:** SCHEDULE 13G

**BUSINESS ADDRESS:**
- **STREET 1:** 600 WASHINGTON BLVD.
- **STREET 2:** FLOOR 11
- **CITY:** STAMFORD
- **STATE:** CT
- **ZIP:** 06901
- **BUSINESS PHONE:** 212-672-7050

**MAIL ADDRESS:**
- **STREET 1:** 600 WASHINGTON BLVD.
- **STREET 2:** FLOOR 11
- **CITY:** STAMFORD
- **STATE:** CT
- **ZIP:** 06901

## Ex-99

**EXHIBIT A - JOINT FILING AGREEMENT**

This joint filing agreement is made and entered into as of this 1st day of May, 2026, by and among Viking Global Investors LP, Viking Global Opportunities Parent GP LLC, Viking Global Opportunities GP LLC, Viking Global Opportunities Portfolio GP LLC, Viking Global Opportunities Illiquid Investments Sub-Master LP, Viking Global Opportunities Vintage IV GP LLC, Viking Global Opportunities Vintage IV Portfolio GP LLC, Viking Global Opportunities Vintage IV (Aggregator) LP, O. Andreas Halvorsen, and Rose S. Shabet.

The parties to this Agreement hereby agree to prepare jointly and file timely (and otherwise to deliver as appropriate) all filings on any Forms 3, 4, or 5 or Schedules 13D or 13G, and any and all amendments thereto and any other documents relating thereto (collectively, the "Filings") as required to be filed pursuant to the Securities Exchange Act of 1934, as amended. The parties to this Agreement further agree and covenant that each will fully cooperate with such other parties in the preparation, timely filing, and delivery of all such Filings.

IN WITNESS WHEREOF, the parties hereto have executed this agreement as of the date first set forth above.

Dated: May 1, 2026

---

| | |
|:---|:---|
| By:  | /s/ Scott M. Hendler  |
| Name:  | Scott M. Hendler on behalf of O. Andreas Halvorsen (1)  |
| By:  | /s/ Scott M. Hendler  |
| Name:  | Scott M. Hendler on behalf of Rose S. Shabet (2)  |

---

(1) Scott M. Hendler is signing on behalf of O. Andreas Halvorsen, individually and as an Executive Committee Member of VIKING GLOBAL PARTNERS LLC, on behalf of VIKING GLOBAL INVESTORS LP, and as an Executive Committee Member of VIKING GLOBAL OPPORTUNITIES PARENT GP LLC, on behalf of itself and VIKING GLOBAL OPPORTUNITIES GP LLC, VIKING GLOBAL OPPORTUNITIES PORTFOLIO GP LLC, VIKING GLOBAL OPPORTUNITIES ILLIQUID INVESTMENTS SUB-MASTER LP, VIKING GLOBAL OPPORTUNITIES VINTAGE IV GP LLC, VIKING GLOBAL OPPORTUNITIES VINTAGE IV PORTFOLIO GP LLC and VIKING GLOBAL OPPORTUNITIES VINTAGE IV (AGGREGATOR) LP, pursuant to an authorization and designation letter dated February 9, 2021, which was previously filed with the Commission as an exhibit to a Form 13G filed by Mr. Halvorsen on February 12, 2021 (SEC File No. 005-49737).

(2) Scott M. Hendler is signing on behalf of Rose S. Shabet, individually and as an Executive Committee Member of VIKING GLOBAL PARTNERS LLC, on behalf of VIKING GLOBAL INVESTORS LP, and as an Executive Committee Member of VIKING GLOBAL OPPORTUNITIES PARENT GP LLC, on behalf of itself and VIKING GLOBAL OPPORTUNITIES GP LLC, VIKING GLOBAL OPPORTUNITIES PORTFOLIO GP LLC, VIKING GLOBAL OPPORTUNITIES ILLIQUID INVESTMENTS SUB-MASTER LP, VIKING GLOBAL OPPORTUNITIES VINTAGE IV GP LLC, VIKING GLOBAL OPPORTUNITIES VINTAGE IV PORTFOLIO GP LLC and VIKING GLOBAL OPPORTUNITIES VINTAGE IV (AGGREGATOR) LP, pursuant to an authorization and designation letter dated February 9, 2021, which was previously filed with the Commission as an exhibit to a Form 13G filed by Mr. Halvorsen on February 12, 2021 (SEC File No. 005-49737).

### UNITED STATES SECURITIES AND EXCHANGE COMMISSION
**Washington, D.C. 20549**

## SCHEDULE 13G

### Under the Securities Exchange Act of 1934

**Issuer:** Altimmune, Inc.

**Title of Class of Securities:** Common Stock, par value $0.0001 per share

**CUSIP Number:** —

**Date of Event Which Requires Filing of this Statement:** 04/24/2026

**Check the appropriate box to designate the rule pursuant to which this Schedule is filed:**

- [ ] Rule 13d-1(b)

- [x] Rule 13d-1(c)

- [ ] Rule 13d-1(d)

---

| 1. | Names of Reporting Persons<br>VIKING GLOBAL INVESTORS LP | Names of Reporting Persons<br>VIKING GLOBAL INVESTORS LP |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>DE | Citizenship or Place of Organization<br>DE |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>13400000 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>13400000 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>13400000 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>13400000 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>6.7% | Percent of Class Represented by Amount in Row (9)<br>6.7% |
| 12. | Type of Reporting Person (See Instructions)<br>PN | Type of Reporting Person (See Instructions)<br>PN |

**Comment for Type of Reporting Person:** See Item 4

---

| 1. | Names of Reporting Persons<br>Viking Global Opportunities Parent GP LLC | Names of Reporting Persons<br>Viking Global Opportunities Parent GP LLC |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>DE | Citizenship or Place of Organization<br>DE |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>13400000 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>13400000 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>13400000 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>13400000 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>6.7% | Percent of Class Represented by Amount in Row (9)<br>6.7% |
| 12. | Type of Reporting Person (See Instructions)<br>OO | Type of Reporting Person (See Instructions)<br>OO |

**Comment for Type of Reporting Person:** See Item 4

---

| 1. | Names of Reporting Persons<br>Viking Global Opportunities GP LLC | Names of Reporting Persons<br>Viking Global Opportunities GP LLC |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>DE | Citizenship or Place of Organization<br>DE |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>9380000 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>9380000 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>9380000 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>9380000 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>4.7% | Percent of Class Represented by Amount in Row (9)<br>4.7% |
| 12. | Type of Reporting Person (See Instructions)<br>OO | Type of Reporting Person (See Instructions)<br>OO |

**Comment for Type of Reporting Person:** See Item 4

---

| 1. | Names of Reporting Persons<br>Viking Global Opportunities Portfolio GP LLC | Names of Reporting Persons<br>Viking Global Opportunities Portfolio GP LLC |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>DE | Citizenship or Place of Organization<br>DE |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>9380000 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>9380000 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>9380000 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>9380000 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>4.7% | Percent of Class Represented by Amount in Row (9)<br>4.7% |
| 12. | Type of Reporting Person (See Instructions)<br>OO | Type of Reporting Person (See Instructions)<br>OO |

**Comment for Type of Reporting Person:** See Item 4

---

| 1. | Names of Reporting Persons<br>Viking Global Opportunities Illiquid Investments Sub-Master LP | Names of Reporting Persons<br>Viking Global Opportunities Illiquid Investments Sub-Master LP |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>E9 | Citizenship or Place of Organization<br>E9 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>9380000 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>9380000 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>9380000 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>9380000 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>4.7% | Percent of Class Represented by Amount in Row (9)<br>4.7% |
| 12. | Type of Reporting Person (See Instructions)<br>PN | Type of Reporting Person (See Instructions)<br>PN |

**Comment for Type of Reporting Person:** See Item 4

---

| 1. | Names of Reporting Persons<br>Viking Global Opportunities Vintage IV GP LLC | Names of Reporting Persons<br>Viking Global Opportunities Vintage IV GP LLC |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>DE | Citizenship or Place of Organization<br>DE |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>4020000 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>4020000 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>4020000 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>4020000 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>2% | Percent of Class Represented by Amount in Row (9)<br>2% |
| 12. | Type of Reporting Person (See Instructions)<br>OO | Type of Reporting Person (See Instructions)<br>OO |

**Comment for Type of Reporting Person:** See Item 4

---

| 1. | Names of Reporting Persons<br>Viking Global Opportunities Vintage IV Portfolio GP LLC | Names of Reporting Persons<br>Viking Global Opportunities Vintage IV Portfolio GP LLC |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>DE | Citizenship or Place of Organization<br>DE |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>4020000 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>4020000 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>4020000 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>4020000 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>2% | Percent of Class Represented by Amount in Row (9)<br>2% |
| 12. | Type of Reporting Person (See Instructions)<br>OO | Type of Reporting Person (See Instructions)<br>OO |

**Comment for Type of Reporting Person:** See Item 4

---

| 1. | Names of Reporting Persons<br>Viking Global Opportunities Vintage IV (Aggregator) LP | Names of Reporting Persons<br>Viking Global Opportunities Vintage IV (Aggregator) LP |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>E9 | Citizenship or Place of Organization<br>E9 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>4020000 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>4020000 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>4020000 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>4020000 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>2% | Percent of Class Represented by Amount in Row (9)<br>2% |
| 12. | Type of Reporting Person (See Instructions)<br>PN | Type of Reporting Person (See Instructions)<br>PN |

**Comment for Type of Reporting Person:** See Item 4

---

| 1. | Names of Reporting Persons<br>HALVORSEN OLE ANDREAS | Names of Reporting Persons<br>HALVORSEN OLE ANDREAS |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>Q8 | Citizenship or Place of Organization<br>Q8 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>13400000 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>13400000 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>13400000 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>13400000 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>6.7% | Percent of Class Represented by Amount in Row (9)<br>6.7% |
| 12. | Type of Reporting Person (See Instructions)<br>IN | Type of Reporting Person (See Instructions)<br>IN |

**Comment for Type of Reporting Person:** See Item 4

---

| 1. | Names of Reporting Persons<br>Shabet Rose Sharon | Names of Reporting Persons<br>Shabet Rose Sharon |
|:---|:---|:---|:---|
| 2. | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] | Check the Appropriate Box if a Member of a Group (See Instructions)<br>(a) [ ] (b) [x] |
| 3. | SEC Use Only | SEC Use Only |
| 4. | Citizenship or Place of Organization<br>X1 | Citizenship or Place of Organization<br>X1 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 5. | Sole Voting Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 6. | Shared Voting Power<br>13400000 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 7. | Sole Dispositive Power<br>0 |
| Number of<br>Shares<br>Beneficially<br>Owned by<br>Each<br>Reporting<br>Person<br>With | 8. | Shared Dispositive Power<br>13400000 |
| 9. | Aggregate Amount Beneficially Owned by Each Reporting Person<br>13400000 | Aggregate Amount Beneficially Owned by Each Reporting Person<br>13400000 |
| 10. | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] | Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) [ ] |
| 11. | Percent of Class Represented by Amount in Row (9)<br>6.7% | Percent of Class Represented by Amount in Row (9)<br>6.7% |
| 12. | Type of Reporting Person (See Instructions)<br>IN | Type of Reporting Person (See Instructions)<br>IN |

**Comment for Type of Reporting Person:** See Item 4

---

**Item 1(a). Name of Issuer:**
Altimmune, Inc.

**Item 1(b). Address of Issuer's Principal Executive Offices:**
910 Clopper Road, Suite 201S, Gaithersburg, Maryland 20878

**Item 2(a). Name of Person Filing:**
Viking Global Investors LP ("VGI"),
Viking Global Opportunities Parent GP LLC ("Opportunities Parent"),
Viking Global Opportunities GP LLC ("Opportunities GP"),
Viking Global Opportunities Portfolio GP LLC ("Opportunities Portfolio GP"),
Viking Global Opportunities Illiquid Investments Sub-Master LP ("VGOP"),
Viking Global Opportunities Vintage IV GP LLC ("VGOVIV GP"),
Viking Global Opportunities Vintage IV Portfolio GP LLC ("VGOVIV Portfolio GP"),
Viking Global Opportunities Vintage IV (Aggregator) LP ("VGOVIV"),
O. Andreas Halvorsen and Rose S. Shabet (collectively, the "Reporting Persons")

**Item 2(b). Address of Principal Business Office:**
The business address of each of the Reporting Persons is: 600 Washington Boulevard, Floor 11, Stamford, Connecticut 06901.

**Item 2(c). Citizenship:**
VGI is a Delaware limited partnership; Opportunities Parent, Opportunities GP, Opportunities Portfolio GP, VGOVIV GP and VGOVIV Portfolio GP are Delaware limited liability companies; VGOP and VGOVIV are Cayman Islands exempted limited partnerships; O. Andreas Halvorsen is a citizen of Norway; and Rose S. Shabet is a citizen of the United States.

**Item 2(d). Title of Class of Securities:**
Common Stock, par value $0.0001 per share

**Item 2(e). CUSIP Number:**
—

**Item 3. If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:**

(a) [ ] Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o).

(b) [ ] Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c).

(c) [ ] Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c).

(d) [ ] Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8).

(e) [ ] An investment adviser in accordance with § 240.13d-1(b)(1)(ii)(E);

(f) [ ] An employee benefit plan or endowment fund in accordance with § 240.13d-1(b)(1)(ii)(F);

(g) [ ] A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);

(h) [ ] A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);

(i) [ ] A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);

(j) [ ] A non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J), if filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J), please specify the type of institution:

(k) [ ] Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K).

**Item 4. Ownership:**

**(a) Amount beneficially owned:**

This Statement on Schedule 13G relates to shares of Common Stock and warrants to purchase shares of Common Stock at an exercise price of $3.00 per share (the "Warrants") held directly by VGOP and VGOVIV.
VGI: 13,400,000
VGI provides managerial services to VGOP and VGOVIV. VGI has the authority to dispose of and vote the shares of Common Stock that VGOP and VGOVIV directly own or have the right to purchase upon exercise of the Warrants.  VGI does not directly own any shares of Common Stock or Warrants.
Based on Rule 13d-3 under the Securities Exchange Act of 1934, as amended (the "Act"), VGI may be deemed to beneficially own the shares of Common Stock that VGOP and VGOVIV directly own or have the right to purchase upon exercise of the Warrants.
VGI beneficially owns 13,400,000 shares of Common Stock consisting of (i) 4,690,000 and 2,010,000 shares of Common Stock directly owned by VGOP and VGOVIV, respectively, and (ii) 4,690,000 and 2,010,000 shares of Common Stock beneficially owned by VGOP and VGOVIV, respectively, which VGOP and VGOVIV have the right to purchase upon exercise of the Warrants.
Opportunities Parent: 13,400,000
Opportunities Parent is the sole member of Opportunities GP, which has the authority to dispose of and vote the shares of Common Stock controlled by Opportunities Portfolio GP (which consists of the shares of Common Stock that VGOP directly owns or has the right to purchase upon exercise of the Warrants), and is the sole member of VGOVIV GP, which has the authority to dispose of and vote the shares of Common Stock controlled by VGOVIV Portfolio GP (which consists of the shares of Common Stock that VGOVIV directly owns or has the right to purchase upon exercise of the Warrants). Opportunities Parent does not directly own any shares of Common Stock or Warrants.
Based on Rule 13d-3 under the Act, Opportunities Parent may be deemed to beneficially own the shares of Common Stock that VGOP and VGOVIV directly own or have the right to purchase upon exercise of the Warrants.
Opportunities Parent beneficially owns 13,400,000 shares of Common Stock consisting of (i) 4,690,000 and 2,010,000 shares of Common Stock directly owned by VGOP and VGOVIV, respectively, and (ii) 4,690,000 and 2,010,000 shares of Common Stock beneficially owned by VGOP and VGOVIV, respectively, which VGOP and VGOVIV have the right to purchase upon exercise of the Warrants.
Opportunities GP: 9,380,000
Opportunities GP serves as the sole member of Opportunities Portfolio GP and has the authority to dispose of and vote the shares of Common Stock controlled by Opportunities Portfolio GP, which consists of the shares of Common Stock that VGOP directly owns or has the right to purchase upon exercise of the Warrants. In addition, Opportunities GP is the general partner of each of Viking Global Opportunities Intermediate LP, Viking Global Opportunities III LP, and Viking Global Opportunities LP. Opportunities GP does not directly own any shares of Common Stock or Warrants.
Based on Rule 13d-3 under the Act, Opportunities GP may be deemed to beneficially own the shares of Common Stock that VGOP directly owns or has the right to purchase upon exercise of the Warrants.
Opportunities GP beneficially owns 9,380,000 shares of Common Stock consisting of 4,690,000 shares of Common Stock directly owned by VGOP and 4,690,000 shares of Common Stock beneficially owned by VGOP, which VGOP has the right to purchase upon exercise of the Warrants.
Opportunities Portfolio GP: 9,380,000
Opportunities Portfolio GP serves as the general partner of VGOP and has the authority to dispose of and vote the shares of Common Stock that VGOP directly owns or has the right to purchase upon exercise of the Warrants. Opportunities Portfolio GP does not directly own any shares of Common Stock or Warrants.
Based on Rule 13d-3 under the Act, Opportunities Portfolio GP may be deemed to beneficially own the shares of Common Stock that VGOP directly owns or has the right to purchase upon exercise of the Warrants.
Opportunities Portfolio GP beneficially owns 9,380,000 shares of Common Stock consisting of 4,690,000 shares of Common Stock directly owned by VGOP and 4,690,000 shares of Common Stock beneficially owned by VGOP, which VGOP has the right to purchase upon exercise of the Warrants.
VGOP: 9,380,000
VGOP has the authority to dispose of and vote the 4,690,000 shares of Common Stock that it owns and the 4,690,000 shares of Common Stock it has the right to purchase upon exercise of the Warrants directly owned by it, which power may be exercised by its general partner, Opportunities Portfolio GP, and by VGI, an affiliate of Opportunities Portfolio GP, which provides managerial services to VGOP.
Viking Global Opportunities LP (a Delaware limited partnership) and Viking Global Opportunities III LP (a Cayman Islands exempted limited partnership), through its investment in Viking Global Opportunities Intermediate LP (a Cayman Islands exempted limited partnership), invest substantially all of their assets in Viking Global Opportunities Master LP (a Cayman Islands exempted limited partnership), which in turn invests through VGOP.
VGOVIV GP: 4,020,000
VGOVIV GP serves as the sole member of VGOVIV Portfolio GP and has the authority to dispose of and vote the shares of Common Stock controlled by VGOVIV Portfolio GP, which consists of the shares of Common Stock that VGOVIV directly owns or has the right to purchase upon exercise of the Warrants. In addition, VGOVIV GP is the general partner of each of Viking Global Opportunities Vintage IV (Offshore) LP and Viking Global Opportunities Vintage IV (Onshore) LP. VGOVIV GP does not directly own any shares of Common Stock or Warrants.
Based on Rule 13d-3 under the Act, VGOVIV GP may be deemed to beneficially own the shares of Common Stock that VGOVIV directly owns or has the right to purchase upon exercise of the Warrants.
VGOVIV GP beneficially owns 4,020,000 shares of Common Stock consisting of 2,010,000 shares of Common Stock directly owned by VGOVIV and 2,010,000 shares of Common Stock beneficially owned by VGOVIV, which VGOVIV has the right to purchase upon exercise of the Warrants.
VGOVIV Portfolio GP: 4,020,000
VGOVIV Portfolio GP serves as the general partner of VGOVIV and has the authority to dispose of and vote the shares of Common Stock that VGOVIV directly owns or has the right to purchase upon exercise of the Warrants. VGOVIV Portfolio GP does not directly own any shares of Common Stock or Warrants.
Based on Rule 13d-3 under the Act, VGOVIV Portfolio GP may be deemed to beneficially own the shares of Common Stock that VGOVIV directly owns or has the right to purchase upon exercise of the Warrants.
VGOVIV Portfolio GP beneficially owns 4,020,000 shares of Common Stock consisting of 2,010,000 shares of Common Stock directly owned by VGOVIV and 2,010,000 shares of Common Stock beneficially owned by VGOVIV, which VGOVIV has the right to purchase upon exercise of the Warrants.
VGOVIV: 4,020,000
VGOVIV has the authority to dispose of and vote the 2,010,000 shares of Common Stock that it directly owns and the 2,010,000 shares of Common Stock it has the right to purchase upon exercise of the Warrants directly owned by it, which power may be exercised by VGOVIV Portfolio GP, and by VGI, an affiliate of VGOVIV Portfolio GP, which provides managerial services to VGOVIV.
Viking Global Opportunities Vintage IV (Offshore) LP (a Cayman Islands exempted limited partnership) and Viking Global Opportunities Vintage IV (Onshore) LP (a Delaware limited partnership) invest substantially all of their assets in VGOVIV.
O. Andreas Halvorsen and Rose S. Shabet: 13,400,000
Mr. Halvorsen and Ms. Shabet, as Executive Committee Members of Viking Global Partners LLC (general partner of VGI) and Opportunities Parent, have shared authority to dispose of and vote the shares of Common Stock beneficially owned by VGI and Opportunities Parent. Neither Mr. Halvorsen nor Ms. Shabet directly owns any shares of Common Stock or Warrants.
Based on Rule 13d-3 under the Act, each may be deemed to beneficially own the shares of Common Stock that VGOP and VGOVIV directly own or have the right to purchase upon exercise of the Warrants.
Each of Mr. Halvorsen and Ms. Shabet beneficially owns 13,400,000 shares of Common Stock consisting of (i) 4,690,000 and 2,010,000 shares of Common Stock directly owned by VGOP and VGOVIV, respectively, and (ii) 4,690,000 and 2,010,000 shares of Common Stock beneficially owned by VGOP and VGOVIV, respectively, which VGOP and VGOVIV have the right to purchase upon exercise of the Warrants.

**(b) Percent of class:**

The percentages set forth herein are based on (i) 194,199,358 shares of Common Stock outstanding following the Issuer's underwritten offering that closed on April 24, 2026, as reported in the Issuer's prospectus on Form 424B5, filed with the Securities and Exchange Commission (the "Commission") on April 24, 2026, and (ii) 6,700,000 shares of Common Stock that the Reporting Persons currently have the right to purchase upon exercise of the Warrants.
VGI: 6.7%
Opportunities Parent: 6.7%
Opportunities GP: 4.7%
Opportunities Portfolio GP: 4.7%
VGOP: 4.7%
VGOVIV GP: 2.0%
VGOVIV Portfolio GP: 2.0%
VGOVIV: 2.0%
O. Andreas Halvorsen and Rose S. Shabet: 6.7%

**(c) Number of shares as to which the person has:**

**(i) Sole power to vote or to direct the vote:**

VGI: 0
Opportunities Parent: 0
Opportunities GP: 0
Opportunities Portfolio GP: 0
VGOP: 0
VGOVIV GP: 0
VGOVIV Portfolio GP: 0
VGOVIV: 0
O. Andreas Halvorsen and Rose S. Shabet: 0

**(ii) Shared power to vote or to direct the vote:**

VGI: 13,400,000
Opportunities Parent: 13,400,000
Opportunities GP: 9,380,000
Opportunities Portfolio GP: 9,380,000
VGOP: 9,380,000
VGOVIV GP: 4,020,000
VGOVIV Portfolio GP: 4,020,000
VGOVIV: 4,020,000
O. Andreas Halvorsen and Rose S. Shabet: 13,400,000

**(iii) Sole power to dispose or to direct the disposition of:**

VGI: 0
Opportunities Parent: 0
Opportunities GP: 0
Opportunities Portfolio GP: 0
VGOP: 0
VGOVIV GP: 0
VGOVIV Portfolio GP: 0
VGOVIV: 0
O. Andreas Halvorsen and Rose S. Shabet: 0

**(iv) Shared power to dispose or to direct the disposition of:**

VGI: 13,400,000
Opportunities Parent: 13,400,000
Opportunities GP: 9,380,000
Opportunities Portfolio GP: 9,380,000
VGOP: 9,380,000
VGOVIV GP: 4,020,000
VGOVIV Portfolio GP: 4,020,000
VGOVIV: 4,020,000
O. Andreas Halvorsen and Rose S. Shabet: 13,400,000

**Item 5. Ownership of Five Percent or Less of a Class.**

[x] If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following.

**Item 6. Ownership of More than 5 Percent on Behalf of Another Person.**

The response to Item 4 is incorporated by reference herein.

**Item 7. Identification and Classification of the Subsidiary**

Not Applicable

**Item 8. Identification and Classification of Members of the Group**

Not Applicable

**Item 9. Notice of Dissolution of Group**

Not Applicable

**Item 10. Certification:**

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under ?? 240.14a-11.

### SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

**VIKING GLOBAL INVESTORS LP**

**Date:** 05/01/2026

**By:** /s/ Scott M. Hendler

**Name & Title:** Scott M. Hendler on behalf of VIKING GLOBAL INVESTORS LP (1)(2)

**Viking Global Opportunities Parent GP LLC**

**Date:** 05/01/2026

**By:** /s/ Scott M. Hendler

**Name & Title:** Scott M. Hendler on behalf of Viking Global Opportunities Parent GP LLC (1)(2)

**Viking Global Opportunities GP LLC**

**Date:** 05/01/2026

**By:** /s/ Scott M. Hendler

**Name & Title:** Scott M. Hendler on behalf of Viking Global Opportunities GP LLC (1)(2)

**Viking Global Opportunities Portfolio GP LLC**

**Date:** 05/01/2026

**By:** /s/ Scott M. Hendler

**Name & Title:** Scott M. Hendler on behalf of Viking Global Opportunities Portfolio GP LLC (1)(2)

**Viking Global Opportunities Illiquid Investments Sub-Master LP**

**Date:** 05/01/2026

**By:** /s/ Scott M. Hendler

**Name & Title:** Scott M. Hendler on behalf of Viking Global Opportunities Illiquid Investments Sub-Master LP (1)(2)

**Viking Global Opportunities Vintage IV GP LLC**

**Date:** 05/01/2026

**By:** /s/ Scott M. Hendler

**Name & Title:** Scott M. Hendler on behalf of Viking Global Opportunities Vintage IV GP LLC (1)(2)

**Viking Global Opportunities Vintage IV Portfolio GP LLC**

**Date:** 05/01/2026

**By:** /s/ Scott M. Hendler

**Name & Title:** Scott M. Hendler on behalf of Viking Global Opportunities Vintage IV Portfolio GP LLC (1)(2)

**Viking Global Opportunities Vintage IV (Aggregator) LP**

**Date:** 05/01/2026

**By:** /s/ Scott M. Hendler

**Name & Title:** Scott M. Hendler on behalf of Viking Global Opportunities Vintage IV (Aggregator) LP (1)(2)

**HALVORSEN OLE ANDREAS**

**Date:** 05/01/2026

**By:** /s/ Scott M. Hendler

**Name & Title:** Scott M. Hendler on behalf of O. Andreas Halvorsen (1)

**Shabet Rose Sharon**

**Date:** 05/01/2026

**By:** /s/ Scott M. Hendler

**Name & Title:** Scott M. Hendler on behalf of Rose S. Shabet (2)