X0306 4 2021-09-01 0000875045 BIOGEN INC. BIIB 0001121897 MCDONNELL MICHAEL R 225 BINNEY STREET CAMBRIDGE MA 02142 1 EVP, Chief Financial Officer Common Stock 2021-09-01 4 M 0 2681 338.24 A 2681 D Common Stock 2021-09-01 4 F 0 1141 338.24 D 1540 D Common Stock 2021-09-01 4 M 0 3324 338.24 A 4864 D Common Stock 2021-09-01 4 F 0 1441 338.24 D 3423 D Restricted Stock Unit 0 2021-09-01 4 M 0 2681 0 D 2023-09-01 Common Stock 2681 5364 D Restricted Stock Unit 0 2021-09-01 4 M 0 3324 0 D 2023-09-01 Common Stock 3324 12766 D Restricted Stock Unit 0 2021-09-01 4 J 0 2038 0 D 2023-09-01 Common Stock 2038 10728 D The restricted stock units vest in three (3) equal annual installments commencing one year after the grant date of 09/01/2020. The number of RSUs reported represents the maximum possible number of shares that are eligible for vesting, which is 200% of the number of shares at target payout. One-third of these RSUs are eligible to vest on each of the first three anniversaries of the grant date. The actual number of shares that will vest on each vesting date will be determined by comparing the price of Biogen common stock on such vesting date to the price on the grant date (i.e., number of vested shares = number of shares at target payout times [the average closing stock price for the 30 calendar days following and including the grant date and 30 calendar days prior to and including such vesting date]). The number of RSUs reported represents the maximum possible number of shares that are eligible for vesting, which is 200% of the number of shares at target payout. One-third of these RSUs are eligible to vest on each of the first three anniversaries of the grant date. The actual number of shares that will vest on each vesting date will be determined by comparing the price of Biogen common stock on such vesting date to the price on the grant date (i.e., number of vested shares = number of shares at target payout times [the 30-day average closing stock price ending on the vesting date divided by the 30-day average closing stock price following and including the grant date]). /s/ Wendell Taylor, attorney-in-fact for Michael R. McDonnell 2021-09-03