SC 13G

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934 (Amendment No.     )*

Arsanis, Inc.

(Name of Issuer)

Common Stock, $0.001 par value per share

(Title of Class of Securities)

042873109

(CUSIP Number)

December 31, 2017

(Date of Event Which Requires Filing of this Statement)

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

☐	Rule 13d-1(b)
☐	Rule 13d-1(c)
☒	Rule 13d-1(d)

*The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

CUSIP No.  042873109

	1.	Names of Reporting Persons Bill & Melinda Gates Foundation
	2.	Check the Appropriate Box if a Member of a Group (See Instructions)
		(a)		☐
		(b)		☐
	3.	SEC Use Only
	4.	Citizenship or Place of Organization  State of Washington
Number ofSharesBeneficiallyOwned byEachReporting Person With:			5.		Sole Voting Power  -0-
			6.		Shared Voting Power 722,179(1)
			7.		Sole Dispositive Power  -0-
			8.		Shared Dispositive Power 722,179(1)
	9.	Aggregate Amount Beneficially Owned by Each Reporting Person722,179(1)
	10.	Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) ☐
	11.	Percent of Class Represented by Amount in Row (9)5.1% (2)
	12.	Type of Reporting Person (See Instructions) OO

_____________________________________

(1)	For purposes of Rule 13d-3 under the Securities Exchange Act of 1934, as amended, all shares of Common Stock (the “Common Stock”) of Arsanis, Inc. (the “Issuer”) beneficially owned by the Bill & Melinda Gates Foundation (the “Foundation”) may be deemed to be beneficially owned by William H. Gates III and Melinda French Gates as Co-Trustees of the Foundation.
(2)	Based on 14,294,383 shares of Common Stock outstanding, as reported in the Issuer’s Prospectus filed with the Securities and Exchange Commission on November 17, 2017 pursuant to Rule 424(b)(1) of the Securities Act of 1933, as amended, and after giving effect to the additional 600,000 shares of Common Stock issued and sold by the Issuer to the underwriters pursuant to the underwriters’ option.

CUSIP No.  042873109

	1.	Names of Reporting Persons William H. Gates III
	2.	Check the Appropriate Box if a Member of a Group (See Instructions)
		(a)		☐
		(b)		☐
	3.	SEC Use Only
	4.	Citizenship or Place of Organization  United States of America
Number ofSharesBeneficiallyOwned byEachReporting Person With:			5.		Sole Voting Power  -0-
			6.		Shared Voting Power 722,179(1)
			7.		Sole Dispositive Power  -0-
			8.		Shared Dispositive Power 722,179(1)
	9.	Aggregate Amount Beneficially Owned by Each Reporting Person722,179(1)
	10.	Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) ☐
	11.	Percent of Class Represented by Amount in Row (9)5.1%(2)
	12.	Type of Reporting Person (See Instructions) IN

_____________________________________

(1)

Bill & Melinda Gates Foundation (the “Foundation”) beneficially owns 722,179 shares of Common Stock (the “Common Stock”) of Arsanis, Inc. (the “Issuer”).  For purposes of Rule 13d-3 under the Securities Exchange Act of 1934, as amended, all shares of Common Stock beneficially owned by the Foundation may be deemed to be beneficially owned by William H. Gates III and Melinda French Gates as Co-Trustees of the Foundation.

(2)

Based on 14,294,383 shares of Common Stock outstanding, as reported in the Issuer’s Prospectus filed with the Securities and Exchange Commission on November 17, 2017 pursuant to Rule 424(b)(1) of the Securities Act of 1933, as amended, and after giving effect to the additional 600,000 shares of Common Stock issued and sold by the Issuer to the underwriters pursuant to the underwriters’ option.

CUSIP No.  042873109

	1.	Names of Reporting Persons Melinda French Gates
	2.	Check the Appropriate Box if a Member of a Group (See Instructions)
		(a)		☐
		(b)		☐
	3.	SEC Use Only
	4.	Citizenship or Place of Organization  United States of America
Number ofSharesBeneficiallyOwned byEachReporting Person With:			5.		Sole Voting Power  -0-
			6.		Shared Voting Power 722,179(1)
			7.		Sole Dispositive Power  -0-
			8.		Shared Dispositive Power 722,179(1)
	9.	Aggregate Amount Beneficially Owned by Each Reporting Person722,179(1)
	10.	Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) ☐
	11.	Percent of Class Represented by Amount in Row (9)5.1%(2)
	12.	Type of Reporting Person (See Instructions) IN

_____________________________________

(1)

Bill & Melinda Gates Foundation (the “Foundation”) beneficially owns 722,179 shares of Common Stock (the “Common Stock”) of Arsanis, Inc. (the “Issuer”).  For purposes of Rule 13d-3 under the Securities Exchange Act of 1934, as amended, all shares of Common Stock beneficially owned by the Foundation may be deemed to be beneficially owned by William H. Gates III and Melinda French Gates as Co-Trustees of the Foundation.

(2)

Based on 14,294,383 shares of Common Stock outstanding, as reported in the Issuer’s Prospectus filed with the Securities and Exchange Commission on November 17, 2017 pursuant to Rule 424(b)(1) of the Securities Act of 1933, as amended, and after giving effect to the additional 600,000 shares of Common Stock issued and sold by the Issuer to the underwriters pursuant to the underwriters’ option.

Item 1.
	(a)	Name of Issuer: Arsanis, Inc. (the “Issuer”)
	(b)	Address of Issuer’s Principal Executive Offices: 890 Winter Street, Suite 230, Waltham, Massachusetts 02451
Item 2.
	(a)	Name of Person Filing:Bill & Melinda Gates Foundation (the “Foundation”), Melinda French Gates, and William H. Gates III (together, the “Reporting Persons”)(1)
	(b)	Address of Principal Business Office or, if none, Residence: Foundation – 500 Fifth Avenue North, Seattle, Washington 98109 Mr. Gates – One Microsoft Way, Redmond, Washington  98052 Mrs. Gates – 500 Fifth Avenue North, Seattle, Washington 98109
	(c)	Citizenship: The Foundation is a charitable trust organized under the laws of the State of Washington. Mr. and Mrs. Gates are citizens of the United States of America.
	(d)	Title of Class of Securities:  Common Stock, $0.001 par value per share.
	(e)	CUSIP Number: 042873109
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:
	Not Applicable.
Item 4.	Ownership.
	Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1.
	(a)	Amount beneficially owned: See the responses to Item 9 on the attached cover pages.
	(b)	Percent of class: See the responses to Item 11 on the attached cover pages.
	(c)	Number of shares as to which the person has:
		(i)	Sole power to vote or to direct the vote See the responses to Item 5 on the attached cover pages.
		(ii)	Shared power to vote or to direct the vote See the responses to Item 6 on the attached cover pages.
		(iii)	Sole power to dispose or to direct the disposition of See the responses to Item 7 on the attached cover pages.
		(iv)	Shared power to dispose or to direct the disposition of See the responses to Item 8 on the attached cover pages.

 ⁽¹⁾    Neither the present filing nor anything contained herein shall be construed as an admission that the Reporting Persons constitute a “group” for any purpose and each expressly disclaims membership in a group.

Item 5.	Ownership of Five Percent or Less of a Class.
	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 5 percent of the class of securities, check the following ☐.
Item 6.	Ownership of More than Five Percent on Behalf of Another Person.
	Not Applicable.
Item 7.	Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.
	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.
	Not Applicable.

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.

Date:  February 13, 2018	BILL & MELINDA GATES FOUNDATION
	By:	/s/ Connie Collingsworth
		Name:	Connie Collingsworth
		Title:	Chief Business Operations and Legal Officer
	WILLIAM H. GATES III
	By:	*
		Name:	Connie Collingsworth (1)
		Title:	Attorney-in-fact
	MELINDA FRENCH GATES
	By:	*
		Name:	Connie Collingsworth (1)
		Title:	Attorney-in-fact
	*By:	/s/	Connie Collingsworth
			Connie Collingsworth

_____________________________________

⁽¹⁾Duly authorized under Special Limited Power of Attorney appointing Connie Collingsworth attorney-in-fact, dated February 7, 2018, by and on behalf of William H. Gates III and Melinda French Gates as Co-Trustees, and attached hereto as Exhibit 99.1.

JOINT FILING AGREEMENT

We, the signatories of the statement to which this Joint Filing Agreement is attached, hereby agree that such statement is filed, and any amendments thereto filed by any or all of us, will be filed, on behalf of each of us.

Date:  February 13, 2018	BILL & MELINDA GATES FOUNDATION
	By:	/s/ Connie Collingsworth
		Name:	Connie Collingsworth
		Title:	Chief Business Operations and Legal Officer
	WILLIAM H. GATES III
	By:	*
		Name:	Connie Collingsworth (1)
		Title:	Attorney-in-fact
	MELINDA FRENCH GATES
	By:	*
		Name:	Connie Collingsworth (1)
		Title:	Attorney-in-fact
	*By:	/s/	Connie Collingsworth
			Connie Collingsworth

_____________________________________

⁽¹⁾Duly authorized under Special Limited Power of Attorney appointing Connie Collingsworth attorney-in-fact, dated February 7, 2018, by and on behalf of William H. Gates III and Melinda French Gates as Co-Trustees, and attached hereto as Exhibit 99.1.

EX-99.1

Special Power of Attorney

The undersigned, being the only Co-Trustees of the Bill & Melinda Gates Foundation (the “Foundation”) having the power and authority to manage the investments of the Foundation and its trust estate, do hereby constitute and appoint Connie Collingsworth as the true and lawful attorney of the undersigned, with full power of substitution, and authorize and designate her for the undersigned and in the undersigned’s name, place and stead, in any and all capacities, to execute, acknowledge, deliver and/or file any documents or filings and any amendments thereto made by or on behalf of the Foundation or the undersigned in respect of any securities or other investments held by the Foundation, directly, indirectly or beneficially.  Without limitation, this authorization extends to and includes any questionnaires, required or discretionary reports and any and all documents and forms with any governmental office or agency, whether U.S., foreign, state or local (including, without limitation, the U.S. Securities & Exchange Commission and state securities administrators or commissions), any securities exchange or market (including, without limitation, the Nasdaq Stock Market), as may be required, or as the undersigned determine to be advisable, under applicable laws (including without limitation, the Securities Act of 1933, as amended, or the Securities Exchange Act of 1934, as amended, and rules and regulations under each of such laws), or rules and regulations of any securities exchange or market, and to do and perform all and every act and thing whatsoever requisite and necessary or convenient to be done related to any such securities or other investments, as fully to all intents and purposes as the undersigned might or could do if personally present.  The undersigned acknowledge that the foregoing attorney-in-fact, in serving in such capacity at the request of the undersigned, is not assuming any of the undersigned’s responsibilities to comply with such laws.

This Special Power of Attorney shall remain in full force and effect until withdrawn by the undersigned by written notice to the foregoing attorney-in-fact.

IN WITNESS WHEREOF, the undersigned have caused this Special Power of Attorney to be executed this 7th day of February 2018.

/s/ William H. Gates III

William H. Gates III, Co-Trustee of the Bill &

Melinda Gates Foundation

/s/ Melinda French Gates

Melinda French Gates, Co-Trustee of the Bill &

Melinda Gates Foundation

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STATE OF WASHINGTON	)
	) ss.
COUNTY OF KING	)

I certify that I know or have satisfactory evidence that William H. Gates III is the person who appeared before me, and said person acknowledged that he signed this instrument, on oath stated that he was authorized to execute the instrument and acknowledged it as a Co-Trustee of the Bill & Melinda Gates Foundation to be the free and voluntary act of such party for the uses and purposes mentioned in the instrument.

Dated	2-7-2018	/s/ Diana Blair
		Signature of Notary Public
	Seal or stamp	Diana Blair
		Print Name
		My appointment expires	03-10-2021

STATE OF WASHINGTON	)
	) ss.
COUNTY OF KING	)

I certify that I know or have satisfactory evidence that Melinda French Gates is the person who appeared before me, and said person acknowledged that she signed this instrument, on oath stated that she was authorized to execute the instrument and acknowledged it as a Co-Trustee of the Bill & Melinda Gates Foundation to be the free and voluntary act of such party for the uses and purposes mentioned in the instrument.

Dated	2-7-2018	/s/ Diana Blair
		Signature of Notary Public
	Seal or stamp	Diana Blair
		Print Name
		My appointment expires	03-10-2021

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