Document:

EX-10.5

 Exhibit 10.5 

Effective Date: 
 INCENTIVE
STOCK OPTION 
 Granted by Arsanis, Inc. 

Under the 2011 Stock Incentive Plan 

For valuable consideration, the receipt of which is hereby acknowledged, Arsanis, Inc., a Delaware corporation (hereinafter together with its
subsidiaries, where the context permits, referred to as the “Company”), hereby grants to the Holder named in Schedule A attached hereto the following Incentive Stock Option (the “Option”): 

Section 1. Grant of Option. Subject to the terms and conditions hereinafter set forth, the Holder is hereby given the right and
option to purchase from the Company shares of the Company’s Common Stock, $.001 par value per share (the “Common Stock”). Schedule A attached hereto and hereby incorporated herein sets forth, with respect to the Option, (i) its
expiration date, (ii) its exercise price per share, (iii) the maximum number of shares that the Holder may purchase upon exercise hereof, and (iv) the vesting schedule. It also sets forth applicable conditions that are incorporated
herein. The Option shall terminate in all respects, and all rights and options to purchase shares hereunder shall terminate, ten years from the Effective Date set forth above. The right to purchase shares hereunder shall be cumulative. 

Section 2. Exercise of Option. The Option may be exercised only to the extent it has vested in accordance with Schedule A attached
hereto. Purchase of any shares hereunder shall be made by delivery to the Company of a written notice of exercise specifying the number of shares with respect to which the Option is to be exercised and the address to which the certificate
representing such shares is to be mailed, accompanied by: 
 (i) cash or a certified or bank check or postal money order
payable to the order of the Company for an amount equal to the aggregate exercise price of the part of the Option being exercised; 

(ii) with the consent of the Company, shares of Common Stock of the Company having a fair market value equal to the aggregate
exercise price of the part of the Option being exercised; 
 (iii) with the consent of the Company, a personal recourse note
issued by the Holder to the Company in a principal amount equal to such aggregate exercise price of the part of the Option being exercised and with such other terms, including interest rate and maturity, as the Company may determine in its
discretion; 
 (iv) with the consent of the Company, if the class of Common Stock is registered under the Securities Exchange
Act of 1934 at that time, subject to rules as may be established by the Board of Directors of the Company (the “Board”), a properly executed exercise notice along with irrevocable instructions to a broker to deliver promptly to the Company
cash or a check payable and acceptable to the Company for the aggregate exercise price of the part of the Option being exercised; 

(v) with the consent of the Company, instructions to reduce the number of shares otherwise issuable to the Holder upon the
exercise of the Option by a number of shares of Common Stock having a fair market equal to the aggregate exercise price of the part of the Option being exercised; or 

 (vi) with the consent of the Company, any combination of (i), (ii), (iii),
(iv) and/or (v). 
 For the purpose of the foregoing, the fair market value of the shares of Common Stock which may be delivered to the
Company upon exercise of the Option shall be determined in accordance with procedures adopted by Board. 
 Section 3. Conditions and
Limitations. As a condition precedent to any exercise of the Option, the Holder (or if any other individual or individuals are exercising the Option, such individual or individuals) shall deliver to the Company an investment letter in form and
substance satisfactory to the Company and its counsel which shall contain among other things a statement in writing to the following effects (to the extent then applicable): (i) that the Option is then being exercised for the account of the
Holder and only with a view to investment in, and not for, in connection with or with a view to the disposition of, the shares with respect to which the Option is then being exercised; (ii) that the Holder acknowledges that the rights of first
refusal and repurchase set forth in Section 9 hereof apply to such shares; (iii) that the Holder has been advised that Rule 144 of the Securities and Exchange Commission (the “Commission”), which permits the resale, subject to
various terms and conditions, of small amounts of “restricted securities” (as therein defined) after they have been held for a prescribed period of time, may not now apply to the Company because the Company is not now required to file, and
does not file, current reports under the Securities Exchange Act of 1934 (the “Exchange Act”), nor is there publicly available information concerning the Company substantially equivalent to that which would be available if the Company were
required to file such reports; (iv) that the Holder understands that there is no assurance that the Company will ever become a reporting company under the Exchange Act and that the Company has no obligation to the Holder to do so; (v) that
the Holder and Holder’s representatives have fully investigated the Company and the business and financial conditions concerning it and have knowledge of the Company’s then current corporate activities and financial condition; and
(vi) that the Holder believes that the nature and amount of the shares being purchased are consistent with Holder’s investment objectives, abilities and resources. The restrictions imposed by this Section and any investment representation
made pursuant to this Section shall be inoperative upon the registration with the Commission under the Securities Act of 1933, as amended (the “Securities Act”), of shares subject to the Option or acquired through the exercise of the
Option. 
 Upon the request of the Company or the managing underwriter(s), the Holder shall, in connection with any public offering of
securities of the Company, agree in writing that for a period of 180 days from the effective date of the registration statement for such offering filed with the Securities and Exchange Commission, plus such additional period, not to exceed 18 days,
as may be necessary to enable the underwriter(s) to comply with Conduct Rule 2711(f) of the National Association of Securities Dealers, Inc., the Holder will not sell, make any short sale of, loan, grant any option for the purchase of, or otherwise
dispose of any shares of the Company’s common stock owned or controlled by him. It shall be a condition to any transfer of the Shares prior to an initial public offering of the Company’s common stock that the transferee agree to be bound
by the foregoing lock-up provision. 
 Section 4. Delivery of Shares. Within a reasonable time following the receipt by the
Company of the written notice and payment of the Option price for the shares to be purchased thereunder and, if applicable, the investment letter referred to in Section 3, the Company will deliver or cause to be delivered to the Holder (or if
any other individual or individuals are exercising the Option, to such individual or individuals) at the address specified pursuant to Section 2 hereof a certificate or certificates for the number of shares with respect to which the Option is
then being exercised, registered in the name of the Holder (or the name or names of the individual or individuals exercising the Option, either alone or 

  
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jointly with another person or persons with rights of survivorship, as the individual or individuals exercising the Option shall prescribe in writing to the Company); provided, however, that such
delivery shall be deemed effected for all purposes when a stock transfer agent shall have deposited such certificate or certificates in the United States mail, addressed to the Holder (or such individual or individuals) at the address so specified;
and provided further that if any law, regulation or order of the Commission or other body having jurisdiction in the premises shall require the Company or the Holder (or the individual or individuals exercising the Option) to take any action in
connection with the sale of the shares then being purchased, then, subject to the other provisions of this Section 4, the date on which such sale shall be deemed to have occurred and the date for the delivery of the certificates for such shares
shall be extended for the period necessary to take and complete such action, it being understood that the Company shall have no obligation to take and complete any such action. 

Section 5. Adjustments Upon Changes in Capitalization. The existence of the Option shall not affect in any way the right or power
of the Company or its stockholders to make or authorize any or all adjustments, recapitalizations, reorganizations or other changes in the Company’s capital structure or its business, or any merger or consolidation of the Company, or any issue
of bonds, debentures, preferred or prior preference stock ahead of or affecting the Common Stock or the rights thereof, or the dissolution or liquidation of the Company, or any sale or transfer of all or any part of its assets or business, or any
other corporate act or proceeding, whether of a similar character or otherwise. 
 If, through or as a result of any merger, consolidation,
sale of all or substantially all of the assets of the Company, reorganization, recapitalization, reclassification, stock dividend, stock split, reverse stock split or other similar transaction, (i) the outstanding shares of Common Stock are
increased, decreased or exchanged for a different number or kind of shares or other securities of the Company, or (ii) additional shares or new or different shares or other securities of the Company or other non-cash assets are distributed with
respect to such shares of Common Stock or other securities, an appropriate and proportionate adjustment shall be made in (x) the number and kind of shares or other securities subject to the Option and (y) the price for each share or other
security subject to the Option, so that upon exercise of the Option, in lieu of the shares of Common Stock for which the Option was then exercisable, the Holder shall be entitled to receive, for the same aggregate cash consideration, the same total
number and kind of shares or other securities, cash or property that the owner of an equal number of outstanding shares of Common Stock immediately prior to the event requiring adjustment would own as a result of the event. If any such event shall
occur, appropriate adjustment shall also be made in the application of the provisions of this Section 5 and Section 6 with respect to the Option and the rights of the Holder after the event so that the provisions of such Sections shall be
applicable after the event and be as nearly equivalent as practicable in operation after the event as they were before the event. 
 Any
adjustments under this Section 5 will be made by the Board, whose determination as to what adjustments, if any, will be made and the extent thereof will be final, binding and conclusive. No fractional shares will be issued under the Option on
account of any such adjustments. 
 Except as hereinbefore expressly provided, the issue by the Company of shares of stock of any class, or
securities convertible into shares of stock of any class, for cash or property, or for labor or services, either upon direct sale or upon the exercise of rights or warrants to subscribe therefor, or upon conversion of shares of obligations of the
Company convertible into such shares or other securities, shall not affect, and no adjustment by reason thereof shall be made with respect to, the number or price of shares of Common Stock then subject to the Option. 

Section 6. Effect of Certain Transactions. If the Company is a party to a merger or reorganization with one or more other
corporations or other entities, whether or not the Company is the surviving or resulting entity, or if the Company consolidates with or into one or more other corporations 

  
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or other entities, or if the Company is liquidated or sells or otherwise disposes of substantially all of its assets (each hereinafter referred to as a “Transaction”), in any case while
this Option remains outstanding, the Board or the board of directors of any entity assuming the obligations of the Company may, in its discretion, 

(i) provide that after the effective date of such Transaction the Option shall remain outstanding and shall be exercisable for
shares of Common Stock or, if applicable, shares of such stock or other securities, cash or property as the holders of shares of Common Stock received pursuant to the terms of such Transaction; 

(ii) accelerate the time for exercise of the Option, so that from and after a date prior to the effective date of such
Transaction the Option shall be exercisable in full; 
 (iii) cancel the Option as of the effective date of the Transaction,
provided that (a) notice of such cancellation shall have been given to the Holder and (b) the Holder shall have the right to exercise the Option to the extent the same is then exercisable or, if the Board shall have accelerated the time
for exercise of the Option, in full during the ten-day period preceding the effective date of the Transaction; or 
 (iv)
determine that in the event of a Transaction under the terms of which holders of Common Stock of the Company receive upon consummation thereof a cash payment for each share surrendered (the “Transaction Price”), the Holder shall be
provided a cash payment equal to the difference between (a) the Transaction Price times the number of shares of Common Stock subject to the Option (to the extent then exercisable at an exercise price that is not in excess of the Transaction
Price) and (b) the aggregate exercise price for all such shares of Common Stock subject to the Option, in exchange for the termination of the Option. 

Section 7. Rights of Holder. No person shall, by virtue of the granting of the Option to the Holder, be deemed to be a holder of
any shares purchasable under the Option or to be entitled to the rights or privileges of a holder of such shares unless and until the Option has been exercised with respect to such shares and they have been issued pursuant to that exercise of the
Option. 
 The granting of the Option shall not impose upon the Company any obligations to employ or to continue to employ the Holder; and
the right of the Company to terminate the employment of the Holder shall not be diminished or affected by reason of the fact that the Option has been granted to the Holder. 

Nothing herein contained shall impose any obligation upon the Holder to exercise the Option. 

Although the Option is intended to qualify as an incentive stock option under the Internal Revenue Code of 1986, as amended (the
“Code”), the Company makes no representation as to the tax treatment to the Holder upon receipt or exercise of the Option or sale or other disposition of the shares covered by the Option. In addition, the Holder understands that for so
long as the Code so provides, the Option shall not constitute an incentive stock option to the extent that the Option and any other incentive stock option of the Company held by the Holder, in the aggregate, become exercisable for the first time in
any one calendar year for shares of Common Stock with an aggregate fair market value determined as of the respective date or dates of grant of the Option(s) of more than $100,000. The Option shall not qualify as an incentive stock option with
respect only to the number of shares which first become exercisable in a calendar year which exceeds such limit, and shall automatically be treated as a non-statutory stock option with respect to such excess shares. To the extent that the Option is
in part an incentive stock option and in part a non-statutory stock option, as contemplated above, the Holder, upon exercise hereof, shall be deemed first to exercise the Option to the extent that it is an incentive stock option, unless the Holder
specifies otherwise in the Holder’s notice of exercise. 

  
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 Section 8. Transfer and Termination. The Option is not transferable by the Holder
otherwise than by will or the laws of descent and distribution. 
 The Option is exercisable, during the Holder’s lifetime, only by
him, and by him only while he is an employee of the Company, except that in the event that the Holder’s employment with the Company terminates for any reason other than death, disability or termination for Cause, the Holder shall have the right
to exercise the Option within a period of ninety (90) days after said termination (but not later than the expiration date of the Option) with respect to the shares which were purchasable by him by exercise of the Option at the time of such
termination of employment. An employment relationship between the Company and the Holder shall be deemed to exist, for purposes of the Option, during any period in which the Holder is employed in any capacity by the Company or any subsidiary of the
Company. 
 In the event of the permanent and total disability or the death of the Holder prior to termination of the Holder’s
employment with the Company or a parent or subsidiary of the Company and before the date of expiration of the Option, the Holder, or in the event of death, his executors, administrators, heirs or legatees, as the case may be, shall have the right to
exercise the Option at any time within one-hundred eighty (180) days after said disability or death (but not after the termination date of the Option) with respect to the shares which were purchasable by the Holder at the date of his disability
or death. The Holder shall be considered permanently and totally disabled if the Holder is disabled within the meaning of Section 22(e)(3) of the Code or any successor provision. 

If the Holder’s employment with the Company is terminated by the Company for Cause, the Option shall immediately terminate and shall
thereafter be of no further force and effect. The term “Cause” shall mean (a) any material breach by the Holder of any agreement to which the Holder and the Company are both parties, (b) any act (other than retirement) or
omission to act by the Holder which may have a material and adverse effect on the Company’s business or on the Holder’s ability to perform services for the Company, including, without limitation, the commission of any crime (other than
minor traffic violations), or (c) any material misconduct or material neglect of duties by the Holder in connection with the business or affairs of the Company or any parent, subsidiary or affiliate of the Company. The Board shall have sole
authority and discretion to determine whether the Holder’s employment has been terminated for Cause. 
 Section 9. Right of
First Refusal. Prior to the effective date of a registration statement under the Securities Act covering any shares of the Company’s Common Stock and until such time as the Company shall have effected a public offering of its Common Stock,
in the event that, at any time when the Holder (which term for purposes of this Section 9 shall mean the Holder and his executors, administrators and any other person to whom this may be transferred by will or the laws of descent and
distribution) is permitted to do so, the Holder desires to sell, assign or otherwise transfer any of the shares issued upon the exercise of the Option, the Holder shall first offer such shares to the Company by giving written notice of the
Holder’s desire so to sell, assign or transfer such shares. The notice shall state the number of shares offered, the name of the person or persons to whom it is proposed to sell, assign or transfer such shares and the price and other terms at
which such shares are intended to be sold, assigned or transferred. Such notice shall constitute an offer to the Company for the Company to purchase the number of shares set forth in the notice at a price per share equal to the price stated therein.
The Company may accept the offer as to all, but not less than all, such shares by notifying the Holder in writing within 30 days after receipt of such notice of its acceptance of the offer. If the offer is accepted, the Company shall have 60 days
within which to purchase the offered shares at a price per share as aforesaid. If within the applicable time periods the Holder does not receive notice of the Company’s 

  
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intention to purchase the offered shares, or if payment in full of the purchase price is not made by the Company, the offer shall be deemed to have been rejected and the Holder may transfer title
to such shares within 90 days from the date of the Holder’s written notice to the Company of the Holder’s intention to sell, but such transfer shall be made only to the proposed transferee and at the proposed price and terms stated in such
notice and after compliance with any other provisions of the Option applicable to the transfer of such shares. Shares that are so transferred to such transferee shall remain subject to the rights of the Company set forth in this Section 9. No
sale, assignment, pledge or transfer of any of the shares covered by the Option shall be effective or given effect on the books of the Company unless all of the applicable provisions of this Section 9 have been duly complied with, and the
Company may inscribe on the face of any certificate representing any of such shares a legend referring to the provisions of this Section. If any transfer of shares is made or attempted in violation of the foregoing restrictions, or if shares are not
offered to the Company as required hereby, the Company shall have the right to purchase such shares from the owner thereof or his transferee at any time before or after the transfer, as herein provided. In addition to any other legal or equitable
remedies which it may have, the Company may enforce its rights by actions for specific performance (to the extent permitted by law) and may refuse to recognize any transferee as one of its stockholders for any purpose, including, without limitation,
for purposes of dividend and voting rights, until all applicable provisions hereof have been complied with. 
 For purposes of the Right of
First Refusal pursuant to this Section 9, the term “shares” shall include, without limitation, all new, substituted or additional securities or other property issued to the Holder by reason of his ownership of Common Stock pursuant to
the exercise of the Option, in connection with any stock dividend, liquidating dividend, stock split or other change in the character or amount of any of the outstanding securities of the Company, or any consolidation, merger or sale of all or
substantially all of the assets of the Company. 
 Any certificate representing shares of stock subject to the provisions of this
Section 9 may have endorsed thereon one or more legends, in addition to any other legends deemed appropriate by the Company, substantially as follows: 

“Any disposition of any interest in the securities represented by this certificate is subject to restrictions, and the securities
represented by this certificate are subject to certain options, contained in a certain agreement between the record holder hereof and the Company, a copy of which will be mailed to any holder of this certificate without charge upon receipt by the
Company of a written request therefor.” 
 The restrictions imposed by this Section 9 shall be in addition to, and not in lieu of,
any restrictions set forth in the Stockholders’ Agreement (as defined below) and shall terminate in all respects upon the effective date of a registration statement under the Securities Act covering the Company’s Common Stock. 

Section 10. Stockholders’ Agreement. Notwithstanding anything to the contrary contained herein, the Holder understands and
hereby agrees that, as a condition to the issuance of shares to the Holder hereunder upon exercise of this Option, the Holder shall become a party to that certain Second Amended and Restated Stockholders’ Agreement among the Company and the
other parties thereto, dated as of April 12, 2016, as the same may be amended or amended and restated from time to time (the “Stockholders’ Agreement”) by executing and delivering an adoption agreement thereto, and that the
Holder shall thereby be bound by, and subject to, all the terms and provisions of the Stockholders’ Agreement as a “Restricted Stockholder”. 

Section 11. Notice. Any notice to be given to the Company hereunder shall be deemed sufficient if addressed to the Company and
delivered to the office of the Company, Arsanis, Inc., 890 Winter Street, Suite 230, Waltham, MA 02451, attention of the president, or such other address as the Company may hereafter designate. 

  
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 Any notice to be given to the Holder hereunder shall be deemed sufficient if addressed to and
delivered in person to the Holder at his address furnished to the Company or when deposited in the mail, postage prepaid, addressed to the Holder at such address. 

Section 12. Notification of Disqualifying Disposition. The Holder agrees to notify the Company in writing immediately after making
a Disqualifying Disposition of any shares of Common Stock received pursuant to the exercise of the Option. The Holder also agrees to provide the Company with any information that the Company shall request concerning any such Disqualifying
Disposition. 
 A “Disqualifying Disposition” shall have the meaning specified in Sections 421(b) and 424(c) of the Code, or any
successor provision; as of the date of grant of the Option a Disqualifying Disposition is any disposition (including any sale) of such shares before the later of (a) the second anniversary of the date of grant of the Option and (b) the
first anniversary of the date on which the Holder acquired such shares by exercising the Option, provided that such holding period requirements terminate upon the death of the Holder. 

The Holder acknowledges that he or she will forfeit the favorable income tax treatment otherwise available with respect to the exercise of the
Option if he or she makes a Disqualifying Disposition of shares received upon exercise of the Option. 
 Section 13. Government and
Other Regulations; Governing Law. The Option is subject to all laws, regulations and orders of any governmental authority which may be applicable thereto and, notwithstanding any of the provisions hereof, the Holder agrees that he will not
exercise the Option granted hereby nor will the Company be obligated to issue any shares of stock hereunder if the exercise thereof or the issuance of such shares, as the case may be, would constitute a violation by the Holder or the Company of any
such law, regulation or order or any provision thereof. Without limiting the generality of the foregoing, the Company shall not be obligated to issue any such shares if in the Company’s sole judgment to do so would cause the Company or such
issue not to be in compliance with the requirements of the Securities Act. The Company shall not be obligated to take any affirmative action in order to cause the exercise of the Option or the issuance of shares pursuant hereto to comply with any
such law, regulation, order or provision. 
 The Option is and shall be subject in every respect to the provisions of the Company’s
2011 Stock Incentive Plan, as amended from time to time, which is incorporated herein by reference and made a part hereof. The Holder hereby accepts the Option subject to all the terms and provisions of the Plan and agrees that (a) in the event
of any conflict between the terms hereof and those of the Plan, the latter shall prevail, and (b) all decisions under and interpretations of the Plan by the Board or the Committee (as defined in the Plan) shall be final, binding and conclusive
upon the Holder and his heirs, legal representatives, successors and permitted assigns. 
 The Option shall be governed by and construed in
accordance with the laws of the State of Delaware. 
 Section 14. Effective Date. The Option shall be effective on the Effective
Date set forth on page 1 hereof. 
 IN WITNESS WHEREOF, the parties have executed the Option as of the Effective Date. 

  
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		 		 	Arsanis, Inc.
				
		 		 	By:	 	  

				
	Acknowledged and accepted:	 		 		 	
	  
	 		 		 	
	                        Holder	 		 		 	

  
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 SCHEDULE A 

Arsanis, Inc. 
 Incentive Stock
Option Granted Under the 
 2011 Stock Incentive Plan 
  

	1.	Name of Holder: 

  

	2.	Date of Grant: 

  

	3.	“Vesting Start Date”: 

  

	4.	Maximum Number of Shares for 

	    	which the Option is Exercisable (“Total Shares”): 

  

	5.	Exercise (purchase) Price per Share (“Option Price”): 

  

	6.	Expiration Date of Option: 

  

	7.	Vesting Schedule: 

  

	8.	All shares purchased upon exercise of the Option are subject to the rights of the Company to repurchase such shares as set forth in Section 9 of the Option, to the agreement to lock up set forth in Section 3
of the Option and to the other terms of the Option and the Plan. 

  
 9EX-10.6

 Exhibit 10.6 

Effective Date:                     

NON-STATUTORY STOCK OPTION 

Granted by Arsanis, Inc. 
 Under
the 2011 Stock Incentive Plan 
 For valuable consideration, the receipt of which is hereby acknowledged, Arsanis, Inc., a Delaware
corporation (hereinafter together with its subsidiaries, where the context permits, referred to as the “Company”), hereby grants to the Holder named in Schedule A attached hereto the following Non-Statutory Stock Option (the
“Option”): 
 Section 1. Grant of Option. Subject to the terms and conditions hereinafter set forth, the Holder
is hereby given the right and option to purchase from the Company shares of the Company’s Common Stock, $.001 par value per share (the “Common Stock”). Schedule A attached hereto and hereby incorporated herein sets forth, with respect
to the Option, (i) its expiration date, (ii) its exercise price per share, (iii) the maximum number of shares that the Holder may purchase upon exercise hereof, and (iv) the vesting schedule. It also sets forth applicable
conditions that are incorporated herein. The Option shall terminate in all respects, and all rights and options to purchase shares hereunder shall terminate, ten years from the Effective Date set forth above. The right to purchase shares hereunder
shall be cumulative. 
 Section 2. Exercise of Option. The Option may be exercised only to the extent it has vested in
accordance with Schedule A attached hereto. Purchase of any shares hereunder shall be made by delivery to the Company of a written notice of exercise specifying the number of shares with respect to which the Option is to be exercised and the address
to which the certificate representing such shares is to be mailed, accompanied by: 
 (i) cash or a certified or bank check
or postal money order payable to the order of the Company for an amount equal to the aggregate exercise price of the part of the Option being exercised; 

(ii) with the consent of the Company, shares of Common Stock of the Company having a fair market value equal to the aggregate
exercise price of the part of the Option being exercised; 
 (iii) with the consent of the Company, a personal recourse note
issued by the Holder to the Company in a principal amount equal to such aggregate exercise price of the part of the Option being exercised and with such other terms, including interest rate and maturity, as the Company may determine in its
discretion; 
 (iv) with the consent of the Company, if the class of Common Stock is registered under the Securities Exchange
Act of 1934 at that time, subject to rules as may be established by the Board of Directors of the Company (the “Board”), a properly executed exercise notice along with irrevocable instructions to a broker to deliver promptly to the Company
cash or a check payable and acceptable to the Company for the aggregate exercise price of the part of the Option being exercised; 

 (v) with the consent of the Company, instructions to reduce the number of shares
otherwise issuable to the Holder upon the exercise of the Option by a number of shares of Common Stock having a fair market equal to the aggregate exercise price of the part of the Option being exercised; or 

(vi) with the consent of the Company, any combination of (i), (ii), (iii), (iv) and/or (v). 

For the purpose of the foregoing, the fair market value of the shares of Common Stock which may be delivered to the Company upon exercise of
the Option shall be determined in accordance with procedures adopted by Board. 
 Section 3. Conditions and Limitations.
As a condition precedent to any exercise of the Option, the Holder (or if any other individual or individuals are exercising the Option, such individual or individuals) shall deliver to the Company an investment letter in form and substance
satisfactory to the Company and its counsel which shall contain among other things a statement in writing to the following effects (to the extent then applicable): (i) that the Option is then being exercised for the account of the Holder and
only with a view to investment in, and not for, in connection with or with a view to the disposition of, the shares with respect to which the Option is then being exercised; (ii) that the Holder acknowledges that the rights of first refusal and
repurchase set forth in Section 9 hereof apply to such shares; (iii) that the Holder has been advised that Rule 144 of the Securities and Exchange Commission (the “Commission”), which permits the resale, subject to various terms
and conditions, of small amounts of “restricted securities” (as therein defined) after they have been held for a prescribed period of time, may not now apply to the Company because the Company is not now required to file, and does not
file, current reports under the Securities Exchange Act of 1934 (the “Exchange Act”), nor is there publicly available information concerning the Company substantially equivalent to that which would be available if the Company were required
to file such reports; (iv) that the Holder understands that there is no assurance that the Company will ever become a reporting company under the Exchange Act and that the Company has no obligation to the Holder to do so; (v) that the
Holder and Holder’s representatives have fully investigated the Company and the business and financial conditions concerning it and have knowledge of the Company’s then current corporate activities and financial condition; and
(vi) that the Holder believes that the nature and amount of the shares being purchased are consistent with Holder’s investment objectives, abilities and resources. The restrictions imposed by this Section and any investment representation
made pursuant to this Section shall be inoperative upon the registration with the Commission under the Securities Act of 1933, as amended (the “Securities Act”), of shares subject to the Option or acquired through the exercise of the
Option. 
 Upon the request of the Company or the managing underwriter(s), the Holder shall, in connection with any public offering
of securities of the Company, agree in writing that for a period of 180 days from the effective date of the registration statement for such offering filed with the Securities and Exchange Commission, plus such additional period, not to exceed 18
days, as may be necessary to enable the underwriter(s) to comply with Conduct Rule 2711(f) of 

  
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the National Association of Securities Dealers, Inc., the Holder will not sell, make any short sale of, loan, grant any option for the purchase of, or otherwise dispose of any shares of the
Company’s common stock owned or controlled by him. It shall be a condition to any transfer of the Shares prior to an initial public offering of the Company’s common stock that the transferee agree to be bound by the foregoing lock-up
provision. 
 Section 4. Delivery of Shares. Within a reasonable time following the receipt by the Company of the written
notice and payment of the Option price for the shares to be purchased thereunder and, if applicable, the investment letter referred to in Section 3, the Company will deliver or cause to be delivered to the Holder (or if any other individual or
individuals are exercising the Option, to such individual or individuals) at the address specified pursuant to Section 2 hereof a certificate or certificates for the number of shares with respect to which the Option is then being exercised,
registered in the name of the Holder (or the name or names of the individual or individuals exercising the Option, either alone or jointly with another person or persons with rights of survivorship, as the individual or individuals exercising the
Option shall prescribe in writing to the Company); provided, however, that such delivery shall be deemed effected for all purposes when a stock transfer agent shall have deposited such certificate or certificates in the United States mail, addressed
to the Holder (or such individual or individuals) at the address so specified; and provided further that if any law, regulation or order of the Commission or other body having jurisdiction in the premises shall require the Company or the Holder (or
the individual or individuals exercising the Option) to take any action in connection with the sale of the shares then being purchased, then, subject to the other provisions of this Section 4, the date on which such sale shall be deemed to have
occurred and the date for the delivery of the certificates for such shares shall be extended for the period necessary to take and complete such action, it being understood that the Company shall have no obligation to take and complete any such
action. 
 Section 5. Adjustments Upon Changes in Capitalization. The existence of the Option shall not affect in
any way the right or power of the Company or its stockholders to make or authorize any or all adjustments, recapitalizations, reorganizations or other changes in the Company’s capital structure or its business, or any merger or consolidation of
the Company, or any issue of bonds, debentures, preferred or prior preference stock ahead of or affecting the Common Stock or the rights thereof, or the dissolution or liquidation of the Company, or any sale or transfer of all or any part of its
assets or business, or any other corporate act or proceeding, whether of a similar character or otherwise.  
 If, through or as a
result of any merger, consolidation, sale of all or substantially all of the assets of the Company, reorganization, recapitalization, reclassification, stock dividend, stock split, reverse stock split or other similar transaction, (i) the
outstanding shares of Common Stock are increased, decreased or exchanged for a different number or kind of shares or other securities of the Company, or (ii) additional shares or new or different shares or other securities of the Company or
other non-cash assets are distributed with respect to such shares of Common Stock or other securities, an appropriate and proportionate adjustment shall be made in (x) the number and kind of shares or other securities subject to the Option and
(y) the price for each share or other security subject to the Option, so that upon exercise of the Option, in lieu of the shares of Common Stock for which the Option was then exercisable, the Holder shall be entitled to receive, for the same
aggregate cash consideration, the same total number and kind of shares or 

  
 3 

 
other securities, cash or property that the owner of an equal number of outstanding shares of Common Stock immediately prior to the event requiring adjustment would own as a result of the event.
If any such event shall occur, appropriate adjustment shall also be made in the application of the provisions of this Section 5 and Section 6 with respect to the Option and the rights of the Holder after the event so that the provisions of
such Sections shall be applicable after the event and be as nearly equivalent as practicable in operation after the event as they were before the event. 

Any adjustments under this Section 5 will be made by the Board, whose determination as to what adjustments, if any, will be made and the
extent thereof will be final, binding and conclusive. No fractional shares will be issued under the Option on account of any such adjustments. 

Except as hereinbefore expressly provided, the issue by the Company of shares of stock of any class, or securities convertible into shares of
stock of any class, for cash or property, or for labor or services, either upon direct sale or upon the exercise of rights or warrants to subscribe therefor, or upon conversion of shares of obligations of the Company convertible into such shares or
other securities, shall not affect, and no adjustment by reason thereof shall be made with respect to, the number or price of shares of Common Stock then subject to the Option. 

Section 6. Effect of Certain Transactions. If the Company is a party to a merger or reorganization with one or more other
corporations or other entities, whether or not the Company is the surviving or resulting entity, or if the Company consolidates with or into one or more other corporations or other entities, or if the Company is liquidated or sells or otherwise
disposes of substantially all of its assets (each hereinafter referred to as a “Transaction”), in any case while this Option remains outstanding, the Board or the board of directors of any entity assuming the obligations of the Company
may, in its discretion,  
 (i) provide that after the effective date of such Transaction the Option shall remain
outstanding and shall be exercisable for shares of Common Stock or, if applicable, shares of such stock or other securities, cash or property as the holders of shares of Common Stock received pursuant to the terms of such Transaction; 

(ii) accelerate the time for exercise of the Option, so that from and after a date prior to the effective date of such
Transaction the Option shall be exercisable in full; 
 (iii) cancel the Option as of the effective date of the Transaction,
provided that (a) notice of such cancellation shall have been given to the Holder and (b) the Holder shall have the right to exercise the Option to the extent the same is then exercisable or, if the Board shall have accelerated the time
for exercise of the Option, in full during the ten-day period preceding the effective date of the Transaction; or 
 (iv)
determine that in the event of a Transaction under the terms of which holders of Common Stock of the Company receive upon consummation thereof a cash payment for each share surrendered (the “Transaction Price”), the Holder shall be
provided a cash payment equal to the difference between (a) the Transaction Price times the number of shares of Common Stock subject to the Option (to the extent then exercisable at an 

  
 4 

 
exercise price that is not in excess of the Transaction Price) and (b) the aggregate exercise price for all such shares of Common Stock subject to the Option, in exchange for the termination
of the Option. 
 Section 7. Rights of Holder. No person shall, by virtue of the granting of the Option to the Holder, be
deemed to be a holder of any shares purchasable under the Option or to be entitled to the rights or privileges of a holder of such shares unless and until the Option has been exercised with respect to such shares and they have been issued pursuant
to that exercise of the Option. 
 The granting of the Option shall not impose upon the Company any obligations to employ or to
continue to employ the Holder or, if applicable, to continue the Holder as a director of, or consultant to, the Company; and the right of the Company to terminate the employment or other service of the Holder shall not be diminished or affected by
reason of the fact that the Option has been granted to the Holder. 
 Nothing herein contained shall impose any obligation upon the Holder
to exercise the Option. 
 Section 8. Transfer and Termination. The Option is not transferable by the Holder otherwise
than by will or the laws of descent and distribution. 
 The Option is exercisable, during the Holder’s lifetime, only by him,
and by him only while he is providing services to the Company, whether as an employee, director or consultant, except that in the event that the Holder’s services with the Company terminate for any reason other than death, disability or
termination for Cause, the Holder shall have the right to exercise the Option within a period of ninety (90) days after said termination (but not later than the expiration date of the Option) with respect to the shares which were purchasable by
him by exercise of the Option at the time of such termination of services. The Holder shall be deemed to be providing services for the Company, for purposes of this Option, during any period in which the Holder is providing services as an employee,
director or consultant for the Company or any subsidiary of the Company. 
 In the event of the permanent and total disability or the death
of the Holder prior to termination of the Holder’s services for the Company or a parent or subsidiary of the Company and before the date of expiration of the Option, the Holder, or in the event of death, his executors, administrators, heirs or
legatees, as the case may be, shall have the right to exercise the Option at any time within one-hundred eighty (180) days after said disability or death (but not after the termination date of the Option) with respect to the shares which were
purchasable by the Holder at the date of his disability or death. The Holder shall be considered permanently and totally disabled if the Holder is disabled within the meaning of Section 22(e)(3) of the Internal Revenue Code of 1986, as amended,
or any successor provision. 
 If the Holder’s services for the Company are terminated by the Company for Cause, the Option shall
immediately terminate and shall thereafter be of no further force and effect. The term “Cause” shall mean (a) any material breach by the Holder of any agreement to which the Holder and the Company are both parties, (b) any act
(other than retirement) or omission to act 

  
 5 

 
by the Holder which may have a material and adverse effect on the Company’s business or on the Holder’s ability to perform services for the Company, including, without limitation, the
commission of any crime (other than minor traffic violations), or (c) any material misconduct or material neglect of duties by the Holder in connection with the business or affairs of the Company or any parent, subsidiary or affiliate of the
Company. The Board shall have sole authority and discretion to determine whether the Holder’s services have been terminated for Cause. 

Section 9. Right of First Refusal. Prior to the effective date of a registration statement under the Securities Act
covering any shares of the Company’s Common Stock and until such time as the Company shall have effected a public offering of its Common Stock, in the event that, at any time when the Holder (which term for purposes of this Section 9 shall
mean the Holder and his executors, administrators and any other person to whom this may be transferred by will or the laws of descent and distribution) is permitted to do so, the Holder desires to sell, assign or otherwise transfer any of the shares
issued upon the exercise of the Option, the Holder shall first offer such shares to the Company by giving written notice of the Holder’s desire so to sell, assign or transfer such shares. The notice shall state the number of shares offered, the
name of the person or persons to whom it is proposed to sell, assign or transfer such shares and the price and other terms at which such shares are intended to be sold, assigned or transferred. Such notice shall constitute an offer to the Company
for the Company to purchase the number of shares set forth in the notice at a price per share equal to the price stated therein. The Company may accept the offer as to all, but not less than all, such shares by notifying the Holder in writing within
30 days after receipt of such notice of its acceptance of the offer. If the offer is accepted, the Company shall have 60 days within which to purchase the offered shares at a price per share as aforesaid. If within the applicable time periods the
Holder does not receive notice of the Company’s intention to purchase the offered shares, or if payment in full of the purchase price is not made by the Company, the offer shall be deemed to have been rejected and the Holder may transfer title
to such shares within 90 days from the date of the Holder’s written notice to the Company of the Holder’s intention to sell, but such transfer shall be made only to the proposed transferee and at the proposed price and terms stated in such
notice and after compliance with any other provisions of the Option applicable to the transfer of such shares. Shares that are so transferred to such transferee shall remain subject to the rights of the Company set forth in this Section 9. No
sale, assignment, pledge or transfer of any of the shares covered by the Option shall be effective or given effect on the books of the Company unless all of the applicable provisions of this Section 9 have been duly complied with, and the
Company may inscribe on the face of any certificate representing any of such shares a legend referring to the provisions of this Section. If any transfer of shares is made or attempted in violation of the foregoing restrictions, or if shares are not
offered to the Company as required hereby, the Company shall have the right to purchase such shares from the owner thereof or his transferee at any time before or after the transfer, as herein provided. In addition to any other legal or equitable
remedies which it may have, the Company may enforce its rights by actions for specific performance (to the extent permitted by law) and may refuse to recognize any transferee as one of its stockholders for any purpose, including, without limitation,
for purposes of dividend and voting rights, until all applicable provisions hereof have been complied with. 
 For purposes of the
Right of First Refusal pursuant to this Section 9, the term “shares” shall include, without limitation, all new, substituted or additional securities or other property 

  
 6 

 
issued to the Holder by reason of his ownership of Common Stock pursuant to the exercise of the Option, in connection with any stock dividend, liquidating dividend, stock split or other change in
the character or amount of any of the outstanding securities of the Company, or any consolidation, merger or sale of all or substantially all of the assets of the Company. 

Any certificate representing shares of stock subject to the provisions of this Section 9 may have endorsed thereon one or more legends,
in addition to any other legends deemed appropriate by the Company, substantially as follows: 
 “Any disposition of any interest in the
securities represented by this certificate is subject to restrictions, and the securities represented by this certificate are subject to certain options, contained in a certain agreement between the record holder hereof and the Company, a copy of
which will be mailed to any holder of this certificate without charge upon receipt by the Company of a written request therefor.” 

The restrictions imposed by this Section 9 shall be in addition to, and not in lieu of, any restrictions set forth in the
Stockholders’ Agreement (as defined below) and shall terminate in all respects upon the effective date of a registration statement under the Securities Act covering the Company’s Common Stock. 

Section 10. Stockholders’ Agreement. Notwithstanding anything to the contrary contained herein, the Holder understands
and hereby agrees that, as a condition to the issuance of shares to the Holder hereunder upon exercise of this Option, the Holder shall become a party to that certain Second Amended and Restated Stockholders’ Agreement among the Company and the
other parties thereto, dated as of April 12, 2016, as the same may be amended or amended and restated from time to time (the “Stockholders’ Agreement”) by executing and delivering an adoption agreement thereto, and that the
Holder shall thereby be bound by, and subject to, all the terms and provisions of the Stockholders’ Agreement as a “Restricted Stockholder”. 

Section 11. Notice. Any notice to be given to the Company hereunder shall be deemed sufficient if addressed to the Company
and delivered to the office of the Company, Arsanis, Inc., 890 Winter Street, Suite 230, Waltham, MA 02451, attention of the president, or such other address as the Company may hereafter designate. 

Any notice to be given to the Holder hereunder shall be deemed sufficient if addressed to and delivered in person to the Holder at his address
furnished to the Company or when deposited in the mail, postage prepaid, addressed to the Holder at such address. 

Section 12. Withholding of Taxes. The Holder agrees that the Company may withhold from amounts due to the Holder from the
Company, the appropriate amount of federal, state and local withholding taxes attributable to the Holder’s exercise of this Option. 

At the Holder’s election, with the consent of the Company, the amount required to be withheld may be satisfied, in whole or in part, by
(i) authorizing the Company to withhold from shares of Common Stock to be issued pursuant to the exercise of this Option a number of shares with an aggregate fair market value equal to the withholding amount due with respect to such exercise or
(ii) transferring to the Company a number of shares of Common Stock with an aggregate fair market value equal to the withholding amount so due. 

  
 7 

 The Holder further agrees that, if the Company does not withhold an amount due to the Holder from
the Company sufficient to satisfy the Company’s withholding obligation, the Holder will reimburse the Company, on demand, in cash for the amount underwithheld. 

Section 13. Government and Other Regulations; Governing Law. The Option is subject to all laws, regulations and orders of
any governmental authority which may be applicable thereto and, notwithstanding any of the provisions hereof, the Holder agrees that he will not exercise the Option granted hereby nor will the Company be obligated to issue any shares of stock
hereunder if the exercise thereof or the issuance of such shares, as the case may be, would constitute a violation by the Holder or the Company of any such law, regulation or order or any provision thereof. Without limiting the generality of the
foregoing, the Company shall not be obligated to issue any such shares if in the Company’s sole judgment to do so would cause the Company or such issue not to be in compliance with the requirements of the Securities Act. The Company shall not
be obligated to take any affirmative action in order to cause the exercise of the Option or the issuance of shares pursuant hereto to comply with any such law, regulation, order or provision. 

The Option is and shall be subject in every respect to the provisions of the Company’s 2011 Stock Incentive Plan, as amended from time to
time (the “Plan”), which is incorporated herein by reference and made a part hereof. The Holder hereby accepts the Option subject to all the terms and provisions of the Plan and agrees that (a) in the event of any conflict between the
terms hereof and those of the Plan, the latter shall prevail, and (b) all decisions under and interpretations of the Plan by the Board or the Committee (as defined in the Plan) shall be final, binding and conclusive upon the Holder and his
heirs, legal representatives, successors and permitted assigns. 
 The Option shall be governed by and construed in accordance with the laws
of the State of Delaware. 
 Section 14. Effective Date. The Option shall be effective on the Effective Date set forth on page 1
hereof. 
 [Signature page follows.] 

  
 8 

 IN WITNESS WHEREOF, the parties have executed the Option as of the Effective Date. 

 

							
		 		 	Arsanis, Inc.
				
		 		 	By:	 	  

				
	Acknowledged and accepted:	 		 		 	
				
	  
	 		 		 	
	                                Holder	 		 		 	

  
 9 

 SCHEDULE A 

Arsanis, Inc. 
 Non-Statutory
Stock Option Granted Under the 
 2011 Stock Incentive Plan 
  

	1.	Name of Holder: 

  

	2.	Date of Grant: 

  

	3.	“Vesting Start Date”: 

  

	4.	Maximum Number of Shares for 

	    	which the Option is Exercisable (“Total Shares”): 

  

	5.	Exercise (purchase) Price per Share (“Option Price”): 

  

	6.	Expiration Date of Option: 

  

	7.	Vesting Schedule: 

  

	8.	All shares purchased upon exercise of the Option are subject to the rights of the Company to repurchase such shares as set forth in Section 9 of the Option, to the agreement to lock up set forth in Section 3
of the Option and to the other terms of the Option and the Plan. 

  
 10

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