Form of 3.250% Senior Notes due 2051, in the form of a Global Note bearing a private placement legend

Exhibit 4.3

3.250% SENIOR NOTE DUE 2051

THIS NOTE IS A GLOBAL SECURITY WITHIN THE MEANING OF THE INDENTURE HEREINAFTER REFERRED TO AND IS REGISTERED IN THE NAME OF THE DEPOSITORY TRUST COMPANY, A NEW YORK CORPORATION (“DTC”), OR A NOMINEE OF DTC. THIS NOTE IS EXCHANGEABLE FOR NOTES REGISTERED IN THE NAME OF A PERSON OTHER THAN DTC OR ITS NOMINEE ONLY IN THE LIMITED CIRCUMSTANCES DESCRIBED IN THE INDENTURE, AND MAY NOT BE TRANSFERRED EXCEPT AS A WHOLE BY DTC TO A NOMINEE OF DTC, BY A NOMINEE OF DTC TO DTC OR ANOTHER NOMINEE OF DTC OR BY DTC OR ANY SUCH NOMINEE TO A SUCCESSOR DEPOSITARY OR A NOMINEE OF SUCH A SUCCESSOR DEPOSITARY.

UNLESS THIS GLOBAL SECURITY IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF DTC TO THE ISSUER OR ITS AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE OR PAYMENT, AND ANY GLOBAL SECURITY ISSUED IS REGISTERED IN THE NAME OF CEDE & CO. OR IN SUCH OTHER NAME AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC (AND ANY PAYMENT IS MADE TO CEDE & CO. OR TO SUCH OTHER ENTITY AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC), ANY TRANSFER, PLEDGE OR OTHER USE HEREOF FOR VALUE OR OTHERWISE BY OR TO ANY PERSON IS WRONGFUL INASMUCH AS THE REGISTERED OWNER HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.

THIS NOTE (OR ITS PREDECESSOR) WAS ORIGINALLY ISSUED IN A TRANSACTION EXEMPT, OR NOT SUBJECT TO, FROM REGISTRATION UNDER THE U.S. SECURITIES ACT OF 1933, AS AMENDED (THE “SECURITIES ACT”), AND THIS NOTE MAY NOT BE OFFERED, SOLD, PLEDGED OR OTHERWISE TRANSFERRED IN THE ABSENCE OF SUCH REGISTRATION OR AN APPLICABLE EXEMPTION THEREFROM. EACH PURCHASER OF THIS NOTE IS HEREBY NOTIFIED THAT THE SELLER OF THIS NOTE MAY BE RELYING ON THE EXEMPTION FROM THE PROVISIONS OF SECTION 5 OF THE SECURITIES ACT PROVIDED BY RULE 144A THEREUNDER.

THE HOLDER OF THIS NOTE AGREES FOR THE BENEFIT OF BIOGEN INC. THAT (A) PRIOR TO THE DATE (THE “RESALE RESTRICTION TERMINATION DATE”) WHICH IS ONE YEAR AFTER THE LATER OF THE ORIGINAL ISSUE DATE HEREOF AND THE LAST DATE ON WHICH THE ISSUER OR ANY AFFILIATE OF THE ISSUER WAS THE OWNER OF THIS NOTE (OR ANY PREDECESSOR OF SUCH SECURITY), THIS NOTE MAY BE OFFERED, RESOLD, PLEDGED OR OTHERWISE TRANSFERRED ONLY (I) TO A PERSON WHOM THE SELLER REASONABLY BELIEVES IS A “QUALIFIED INSTITUTIONAL BUYER” (AS DEFINED IN RULE 144A UNDER THE SECURITIES ACT) IN A TRANSACTION MEETING THE REQUIREMENTS OF RULE 144A, (II) PURSUANT TO AN EXEMPTION FROM REGISTRATION UNDER THE SECURITIES ACT PROVIDED BY RULE 144 THEREUNDER (IF AVAILABLE), (III) PURSUANT TO OFFERS AND SALES TO NON-U.S. PERSONS THAT OCCUR OUTSIDE THE U.S. WITHIN THE MEANING OF REGULATION S UNDER THE SECURITIES ACT,

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(IV) TO AN INSTITUTIONAL “ACCREDITED INVESTOR” WITHIN THE MEANING OF SUBPARAGRAPH (a)(1), (2), (3) OR (7) OF RULE 501 UNDER THE SECURITIES ACT THAT IS ACQUIRING THIS NOTE FOR ITS OWN ACCOUNT, OR FOR THE ACCOUNT OF SUCH AN INSTITUTIONAL “ACCREDITED INVESTOR,” FOR INVESTMENT PURPOSES AND NOT WITH A VIEW TO, OR FOR OFFER OR SALE IN CONNECTION WITH, ANY DISTRIBUTION IN VIOLATION OF THE SECURITIES ACT, (V) PURSUANT TO ANY OTHER AVAILABLE EXEMPTION FROM THE REGISTRATION REQUIREMENTS OF THE SECURITIES ACT OR (VI) PURSUANT TO A REGISTRATION STATEMENT WHICH HAS BEEN DECLARED EFFECTIVE UNDER THE SECURITIES ACT, IN EACH CASE IN ACCORDANCE WITH ANY APPLICABLE SECURITIES LAWS OF ANY STATE OF THE U.S., AND (B) THE HOLDER WILL, AND EACH SUBSEQUENT HOLDER IS REQUIRED TO, NOTIFY ANY PURCHASER OF THIS NOTE OF THE RESALE RESTRICTIONS REFERRED TO IN CLAUSE (A) ABOVE. THIS LEGEND WILL BE REMOVED UPON THE EARLIER OF THE TRANSFER OF THIS NOTE PURSUANT TO CLAUSE (A)(VI) ABOVE OR REQUEST OF THE HOLDER AFTER THE RESALE RESTRICTION TERMINATION DATE. THE INDENTURE CONTAINS A PROVISION REQUIRING THE TRUSTEE TO REFUSE TO REGISTER THE TRANSFER OF THIS NOTE IN VIOLATION OF THE FOREGOING RESTRICTION.

BIOGEN INC.

3.250% Senior Note due 2051

CUSIP: 09062X AJ2

ISIN: US09062XAJ28

Biogen Inc., a Delaware corporation (herein called the “Company,” which term includes any successor Person under the Indenture hereinafter referred to), for value received, hereby promises to pay to Cede & Co., or registered assigns, the principal sum of $[ ] on February 15, 2051 (the “Maturity Date”) (except to the extent redeemed or repaid prior to the Maturity Date) and to pay interest thereon from February 16, 2021 (the “Original Issue Date”) or from the most recent Interest Payment Date to which interest has been paid or duly provided for semi-annually at the rate of 3.250% per annum, on February 15 and August 15 of each year (each such date, an “Interest Payment Date”), commencing August 15, 2021, until the principal hereof is paid or made available for payment.

1. Payment of Interest. The interest so payable, and punctually paid or made available for payment, on any Interest Payment Date, will, as provided in the Indenture, be paid, in immediately available funds, to the Person in whose name this Note (or one or more predecessor Securities) is registered at the close of business on February 1 or August 1 (whether or not a Business Day, as defined in the Indenture), as the case may be, next preceding such Interest Payment Date (the “Regular Record Date”). Any such interest not punctually paid or duly provided for (“Defaulted Interest”) will forthwith cease to be payable to the Holder on such Regular Record Date, and such Defaulted Interest may be paid to the Person in whose name this

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Note (or one or more predecessor Securities) is registered at the close of business on a special record date (the “Special Record Date”) for the payment of such Defaulted Interest to be fixed by the Trustee, notice whereof shall be given to Holders of Notes not less than fifteen days prior to such Special Record Date, or may be paid at any time in any other lawful manner not inconsistent with the requirements of any securities exchange on which the Notes may be listed, and upon such notice as may be required by such exchange, all as more fully provided in said Indenture.

2. Place of Payment. Payment of principal, premium, if any, and interest on this Note will be made at the Corporate Trust Office of the Trustee or such other office or agency of the Company as may be designated for such purpose, in such coin or currency of the United States of America as at the time of payment is legal tender for payment of public and private debts; provided, however, that each installment of interest, premium, if any, and principal on this Note may at the Company’s option be paid in immediately available funds by transfer to an account maintained by the payee located in the United States.

3. Time of Payment. In any case where any Interest Payment Date, the Maturity Date or any date fixed for redemption or repayment of the Notes shall not be a Business Day, then (notwithstanding any other provision of the Indenture or this Note), payment of principal or interest, if any, need not be made on such date, but may be made on the next succeeding Business Day with the same force and effect as if made on such Interest Payment Date, the Maturity Date or the date so fixed for redemption or repayment, and no interest shall accrue in respect of the delay.

4. General. This Note is one of a duly authorized issue of Securities of the Company, issued and to be issued in one or more Series under an indenture, dated as of September 15, 2015 (the “Base Indenture”), between the Company and U.S. Bank National Association (herein called the “Trustee,” which term includes any successor trustee under the Indenture with respect to a Series of which this Note is a part), as supplemented by a Third Supplemental Indenture thereto, dated as of February 16, 2021 (the “Third Supplemental Indenture” and, together with the Base Indenture, the “Indenture”). Reference is hereby made to the Indenture for a statement of the respective rights, limitations of rights, duties and immunities thereunder of the Company, the Trustee and the Holders of the Securities, and of the terms upon which the Securities are, and are to be, authenticated and delivered. This Note is one of a duly authorized Series of Securities designated as “3.250% Senior Notes due 2051” (the “Notes”), initially limited in aggregate principal amount to $700,731,000.

5. Further Issuance. The Company may from time to time, without the consent of the Holders of the Notes, issue additional Securities (the “Additional Securities”) of this Series having the same ranking and the same interest rate, maturity and other terms as the Notes. Any Additional Securities of this Series and the Notes will constitute a single Series under the Indenture and all references to the Notes shall include the Additional Securities unless the context otherwise requires.

6. Events of Default. If an Event of Default with respect to the Notes shall have occurred and be continuing, the principal of the Notes may be declared due and payable in the manner and with the effect provided in the Indenture.

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7. Sinking Fund. The Notes are not subject to any sinking fund.

8. Optional Redemption. The Notes will be redeemable at any time, at the option of the Company, in whole or from time to time in part, upon not less than 10 nor more than 60 days’ prior notice, on any date prior to their Maturity Date at a redemption price, calculated pursuant to the Indenture, which includes accrued interest thereon, if any, to, but not including, the Redemption Date. In the case of any partial redemption, selection of the Notes for redemption will be made by the Trustee by such methods as the Trustee in its sole discretion shall deem fair and appropriate. If any Note is to be redeemed in part only, the notice of redemption relating to such Note shall state the portion of the principal amount thereof to be redeemed. A new Note in principal amount equal to the unredeemed portion thereof will be issued in the name of the Holder thereof upon cancellation of this Note.

9. Repurchase upon a Change of Control Triggering Event. Upon the occurrence of a Change of Control Triggering Event with respect to the Notes, the Company shall be required to make an offer to repurchase the Notes on the terms set forth in the Indenture.

10. Defeasance and Covenant Defeasance. The Indenture contains provisions for defeasance at any time of (a) the entire indebtedness of the Company on this Note and (b) certain restrictive covenants and the related Defaults and Events of Default, upon compliance by the Company with certain conditions set forth therein, which provisions apply to this Note.

11. Modification and Waivers; Obligations of the Company Absolute. The Indenture permits, with certain exceptions as therein provided, the amendment thereof and the modification of the rights and obligations of the Company and the rights of the Holders of the Securities of each Series. Such amendment may be effected under the Indenture at any time by the Company and the Trustee with the consent of the Holders of not less than a majority in aggregate principal amount of the outstanding Notes of each Series affected thereby. The Indenture also contains provisions permitting the Holders of not less than a majority in aggregate principal amount of the Securities at the time outstanding, on behalf of the Holders of all outstanding Securities, to waive compliance by the Company with certain provisions of the Indenture. Furthermore, provisions in the Indenture permit the Holders of not less than a majority in aggregate principal amount of the outstanding Securities of individual Series to waive on behalf of all of the Holders of Securities of such individual Series certain past defaults under the Indenture and their consequences. Any such consent or waiver shall be conclusive and binding upon the Holder of this Note and upon all future Holders of this Note and of any Note issued upon the registration of transfer hereof or in exchange hereof or in lieu hereof, whether or not notation of such consent or waiver is made upon this Note.

No reference herein to the Indenture and no provision of this Note or of the Indenture shall alter or impair the obligation of the Company, which is absolute and unconditional, to pay the principal of and interest on this Note at the time, place, and rate, and in the coin or currency, herein prescribed.

12. Limitation on Suits. As set forth in, and subject to, the provisions of the Indenture, no Holder of any Note will have any right to institute any proceeding with respect to the Indenture or for any remedy thereunder, unless such Holder shall have previously given to the

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Trustee written notice of a continuing Event of Default with respect to this Series, the Holders of not less than 25% in aggregate principal amount of the outstanding Notes shall have made written request, and offered indemnity and/or security, to the Trustee to institute such proceedings as trustee, and the Trustee shall not have received from the Holders of a majority in principal amount of the outstanding Notes a direction inconsistent with such request and shall have failed to institute such proceeding within 60 days; provided, however, that such limitations do not apply to a suit instituted by the Holder hereof for the enforcement of payment of the principal of or interest on this Note on or after the respective due dates expressed herein.

13. Authorized Denominations. The Notes are issuable only in registered form without coupons in denominations of $2,000 or any integral multiple of $1,000 in excess thereof. A Holder shall register the transfer of or exchange Securities in accordance with the Indenture. The Company may require a Holder, among other things, to furnish appropriate endorsements and transfer documents and to pay certain transfer taxes or similar governmental charges payable in connection therewith as permitted by the Indenture. The Company need not issue, authenticate, register the transfer of or exchange any Securities or portions thereof for a period of 15 days before such series is selected for redemption, nor need the Company register the transfer or exchange of any Security selected for redemption in whole or in part.

14. Registration of Transfer or Exchange. As provided in the Indenture and subject to certain limitations herein and therein set forth, the transfer of this Note is registrable in the register of the Notes maintained by the Registrar upon surrender of this Note for registration of transfer, at the office or agency of the Company in any place where the principal of and interest on this Note are payable, duly endorsed by, or accompanied by a written instrument of transfer in form satisfactory to the Company and the Registrar, duly executed by the Holder hereof or his attorney duly authorized in writing, and thereupon one or more new Notes, of authorized denominations and for the same aggregate principal amount, will be issued to the designated transferee or transferees.

As provided in the Indenture and subject to certain limitations herein and therein set forth, the Notes are exchangeable for a like aggregate principal amount of Notes of different authorized denominations, as requested by the Holders surrendering the same.

No service charge shall be made for any such registration of transfer or exchange, but the Company may require payment of a sum sufficient to cover any tax or other governmental charge payable in connection therewith.

Prior to due presentment of this Note for registration of transfer, the Company, the Trustee and any agent of the Company or the Trustee may treat the Holder as the owner hereof for all purposes, whether or not this Note be overdue, and neither the Company, the Trustee nor any such agent shall be affected by notice to the contrary.

The Holder of this Note is entitled to the benefits of the Registration Rights Agreement. Capitalized terms used in this paragraph have the meanings given to them in the Registration Rights Agreement. Subject to the terms of the Registration Rights Agreement, if a Registration Default occurs, the interest rate on this Note will be increased by 0.25% per annum from the date such Registration Default occurs until and including the date such Registration Default ends.

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Any such additional interest shall be paid in the same manner and on the same dates as interest payments in respect of this Note. A “Registration Default” shall mean the occurrence of any of the following: (i) the Exchange Offer is not completed on or prior to the Target Registration Date, (ii) the Shelf Registration Statement, if required pursuant to Section 2(b)(i) or Section 2(b)(ii) of the Registration Rights Agreement, has not become effective on or prior to the 180^th day after the Target Registration Date, (iii) if the Company receives a Shelf Request pursuant to Section 2(b)(iii) of the Registration Rights Agreement, the Shelf Registration Statement required to be filed thereby has not become effective by the 180^th day after the later of the Target Registration Date and the receipt of such Shelf Request, or (iv) the Shelf Registration Statement, if required by the Registration Rights Agreement, has become effective and thereafter ceases to be effective or the Prospectus contained therein ceases to be usable for its intended purpose without being succeeded promptly by a post-effective amendment to such Registration Statement that cures such failure and that is itself promptly declared effective, and such failure to remain effective or usable exists for more than 90 days (whether or not consecutive) in any 12-month period. A Registration Default ends when this Note ceases to be a Registrable Security or, if earlier, (1) in the case of a Registration Default under clause (i) of the definition thereof, when the Exchange Offer is completed, (2) in the case of a Registration Default under clause (ii) or clause (iii) of the definition thereof, when the Shelf Registration Statement becomes effective or (3) in the case of a Registration Default under clause (iv) of the definition thereof, when the Shelf Registration Statement again becomes effective or the related Prospectus again becomes usable (or, if earlier, when the Shelf Effectiveness Period ends). This Note shall cease to be a Registrable Security upon the earliest of the following: (i) when the Exchange Offer Registration Statement has become effective and this Note been exchanged for Exchange Securities or otherwise disposed of pursuant to such Registration Statement, (ii) when a Registration Statement with respect to this Note has become effective under the Securities Act and this Note has been disposed of pursuant to such Registration Statement, (iii) when this Note ceases to be outstanding or (iv) unless this Note otherwise remains a Registrable Security and is held by a Holder and is ineligible to be exchanged in the Exchange Offer, when the Exchange Offer is consummated. If at any time more than one Registration Default has occurred and is continuing, then, until the next date that there is no Registration Default, the increase in interest rate provided for by this paragraph shall apply as if there occurred a single Registration Default that begins on the date that the earliest such Registration Default occurred and ends on such next date that there is no Registration Default. This paragraph is subject in all respects to the terms and conditions of the Registration Rights Agreement and does not in any way amend, modify, change or enlarge any rights or obligations under the Registration Rights Agreement.

15. Defined Terms. All terms used in this Note, which are defined in the Indenture and are not otherwise defined herein, shall have the meanings assigned to them in the Indenture.

16. Governing Law. This Note shall be governed by and construed in accordance with the laws of the State of New York.

Unless the certificate of authentication hereon has been executed by the Trustee by manual signature, this Note shall not be entitled to any benefit under the Indenture or be valid or obligatory for any purpose.

[Remainder of page intentionally left blank.]

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IN WITNESS WHEREOF, the Company has caused this instrument to be duly executed and its seal to be hereunto affixed.

Dated: February 16, 2021

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TRUSTEE’S CERTIFICATE OF AUTHENTICATION

This is one of the Notes of the Series designated and referred to in the within-mentioned Indenture, as such is supplemented by the within-mentioned Third Supplemental Indenture.

Dated: February 16, 2021

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ASSIGNMENT

(Please print or typewrite name and address,

including postal zip code, of assignee)

to transfer said Note on the books of the Trustee, with full power of substitution in the premises.

CHECK ONE BOX BELOW

(1) ☐ This Note is being transferred inside the United States to a “qualified institutional buyer” (as defined in Rule 144A under the Securities Act of 1933, as amended (the “Securities Act”)) that purchases for its own account or for the account of a qualified institutional buyer to whom notice is given that such transfer is being made in reliance on Rule 144A, in each case pursuant to and in compliance with Rule 144A under the Securities Act; or

(2) ☐ This Note is being transferred outside the United States to a Non-U.S. Person in an offshore transaction within the meaning of Regulation S under the Securities Act in compliance with Rule 904 under the Securities Act.

Unless one of the boxes is checked, the Trustee will not be obligated to register this Note in the name of any Person other than the Holder hereof unless and until the conditions to any such transfer of registration set forth herein and in Section 2.3 of the Third Supplemental Indenture shall have been satisfied.

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TO BE COMPLETED BY PURCHASER IF (1) ABOVE IS CHECKED.

The undersigned represents and warrants that it is purchasing this Note for its own account or an account with respect to which it exercises sole investment discretion and that each of it and any such account is a “qualified institutional buyer” within the meaning of Rule 144A under the Securities Act, and is aware that the sale to it is being made in reliance on Rule 144A and acknowledges that it has received such information regarding the Company as the undersigned has requested pursuant to Rule 144A or has determined not to request such information and that it is aware that the transferor is relying upon the undersigned’s foregoing representations in order to claim the exemption from registration provided by Rule 144A.

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