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Stock research / United States / BIIB / Short interest

Biogen Inc. short interest

5.5M shares of Biogen were sold short at the 15 October 2026 settlement, near the middle of its range over the last year (3.8M to 6.8M). Short sellers would need 4.4 days of normal trading to buy them back.

Shares sold short5.5Mat 15 Oct 2026
Change on the settlement before-7.8%+6% over a year
Days to cover4.39at 1.3M shares a day
Of shares in issue3.72%shares short over shares outstanding
New to this? How to read this page
  1. Short interest is the number of shares investors have borrowed and sold, betting the price will fall, and not yet bought back.
  2. Days to cover is shares short over an average day's trading: how many days it would take short sellers to buy them all back. Above 5 or so is high.
  3. Rising short interest says more investors are betting against the stock. A crowded short can also rise fast if shorts rush to buy back.
  4. Brokers report it twice a month to FINRA, about ten days after each settlement date.
  5. What is short interest?

Short interest, days to cover and the price

Short interest is near the middle of its range over the last year (3.8M to 6.8M), and the share price moved +73% over the last year.

Shares sold shortDays to coverShare price

Three charts on one timeline. Top: shares sold short at each settlement. Middle: days to cover, taller meaning shorts would need longer to buy back. Bottom: the share price, so you can see whether shorts added as the price fell or rose.

How it compares

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    Every settlement since 29 Dec 2017 as a table
    SettlementShares shortChangeAverage daily volumeDays to cover
    15 October 20265.5M-7.8%1.3M4.39
    30 September 20266.0M+2.5%1.2M4.87
    15 September 20265.8M+1.6%767K7.59
    31 August 20265.7M+10.3%662K8.66
    14 August 20265.2M-4.8%847K6.13
    31 July 20265.5M-9.1%1.2M4.73
    15 July 20266.0M+4.8%1.7M3.45
    30 June 20265.7M+10.3%1.4M4.14
    15 June 20265.2M+8.7%919K5.65
    29 May 20264.8M+0.9%1.1M4.49
    15 May 20264.7M+5.7%1.1M4.21
    30 April 20264.5M+1.4%1.2M3.59
    15 April 20264.4M+15.4%1.4M3.07
    31 March 20263.8M-11.7%1.1M3.47
    13 March 20264.3M+3.1%842K5.15
    27 February 20264.2M-7.5%908K4.63
    13 February 20264.5M-3.1%1.9M2.45
    30 January 20264.7M+0.3%1.3M3.62
    15 January 20264.7M+4.7%1.8M2.62
    31 December 20254.5M-32.2%2.7M1.67
    15 December 20256.6M-2.8%1.8M3.67
    28 November 20256.8M+10.4%2.1M3.26
    14 November 20256.1M+8.8%1.9M3.19
    31 October 20255.6M+8.7%1.6M3.60
    15 October 20255.2M+6.4%1.9M2.69
    30 September 20254.9M-4.8%1.6M2.98
    15 September 20255.1M+8.6%1.7M3.01
    29 August 20254.7M+13.6%1.3M3.58
    15 August 20254.2M+4.9%1.5M2.68
    31 July 20254.0M+1.6%1.4M2.78
    15 July 20253.9M-2.6%1.2M3.23
    30 June 20254.0M+18.0%1.3M3.07
    13 June 20253.4M-10.7%1.3M2.57
    30 May 20253.8M+13.6%1.8M2.13
    15 May 20253.3M+4.9%1.7M1.93
    30 April 20253.2M-7.6%1.5M2.17
    15 April 20253.4M+3.1%2.1M1.68
    31 March 20253.3M+1.0%1.6M2.09
    14 March 20253.3M-11.0%1.9M1.73
    28 February 20253.7M-4.2%1.6M2.30
    14 February 20253.9M+3.8%1.7M2.35
    31 January 20253.7M-0.8%1.3M2.91
    15 January 20253.8M+2.5%1.3M3.01
    31 December 20243.7M+42.7%1.7M2.20
    13 December 20242.6M-5.4%1.3M1.94
    29 November 20242.7M+1.0%1.5M1.76
    15 November 20242.7M-13.6%1.8M1.46
    31 October 20243.1M+1.7%1.2M2.52
    15 October 20243.1M+7.7%1.4M2.17
    30 September 20242.8M-22.3%1.3M2.24
    13 September 20243.7M+9.9%857K4.28
    30 August 20243.3M+24.6%691K4.83
    15 August 20242.7M-2.0%1.1M2.52
    31 July 20242.7M-24.6%1.2M2.36
    15 July 20243.6M+32.3%774K4.69
    28 June 20242.7M-10.6%1.2M2.20
    14 June 20243.1M+1.7%947K3.24
    31 May 20243.0M-13.2%973K3.10
    15 May 20243.5M+10.4%1.0M3.34
    30 April 20243.1M-3.6%1.6M1.97
    15 April 20243.3M-8.1%981K3.33
    28 March 20243.6M+33.6%1.2M2.85
    15 March 20242.7M-4.9%1.4M1.97
    29 February 20242.8M-9.7%1.2M2.26
    15 February 20243.1M+1.3%1.3M2.46
    31 January 20243.1M-18.8%949K3.22
    12 January 20243.8M-3.8%952K3.95
    29 December 20233.9M+32.5%882K4.43
    15 December 20233.0M+4.2%1.4M2.09
    30 November 20232.8M+7.6%909K3.12
    15 November 20232.6M-2.3%1.3M2.10
    31 October 20232.7M+1.9%1.1M2.52
    13 October 20232.6M-7.6%898K2.95
    29 September 20232.9M-10.9%788K3.63
    15 September 20233.2M+2.4%814K3.94
    31 August 20233.1M+3.5%620K5.06
    15 August 20233.0M-11.5%978K3.10
    31 July 20233.4M+32.9%1.7M2.05
    14 July 20232.6M+24.1%1.1M2.29
    30 June 20232.1M-7.2%1.0M2.03
    15 June 20232.2M-11.7%1.2M1.89
    31 May 20232.5M-14.6%803K3.15
    15 May 20233.0M+30.7%1.1M2.65
    28 April 20232.3M+13.2%1.1M2.03
    14 April 20232.0M-2.3%662K3.02
    31 March 20232.0M+3.6%840K2.44
    15 March 20232.0M+7.0%878K2.25
    28 February 20231.8M+8.2%956K1.93
    15 February 20231.7M-8.4%877K1.95
    31 January 20231.9M-1.3%927K2.01
    13 January 20231.9M+10.6%1.5M1.25
    30 December 20221.7M-18.5%942K1.81
    15 December 20222.1M-15.8%1.2M1.68
    30 November 20222.5M+2.7%1.3M1.91
    15 November 20222.4M-2.0%1.3M1.86
    31 October 20222.5M-1.0%1.5M1.61
    14 October 20222.5M-7.9%1.8M1.40
    30 September 20222.7M+30.8%3.1M1.00
    15 September 20222.1M+9.9%908K2.28
    31 August 20221.9M-5.6%675K2.80
    15 August 20222.0M+3.2%807K2.48
    29 July 20221.9M-7.3%1.1M1.84
    15 July 20222.1M-17.1%802K2.61
    30 June 20222.5M-5.2%1.4M1.79
    15 June 20222.7M+10.5%963K2.76
    31 May 20222.4M+9.6%997K2.41
    13 May 20222.2M+7.8%1.4M1.51
    29 April 20222.0M+5.6%1.0M1.94
    14 April 20221.9M-0.1%945K2.04
    31 March 20221.9M-12.3%974K1.98
    15 March 20222.2M-11.3%1.1M2.00
    28 February 20222.5M-2.5%1.1M2.23
    15 February 20222.5M-13.7%1.1M2.31
    31 January 20222.9M+2.0%1.7M1.78
    14 January 20222.9M+17.3%2.3M1.27
    31 December 20212.5M+5.9%2.1M1.20
    15 December 20212.3M-2.5%1.3M1.85
    30 November 20212.4M+16.4%1.5M1.60
    15 November 20212.0M-13.8%979K2.09
    29 October 20212.4M-0.9%1.4M1.75
    15 October 20212.4M-3.7%822K2.92
    30 September 20212.5M+1.8%930K2.68
    15 September 20212.4M+2.9%1.2M1.97
    31 August 20212.4M+9.0%785K3.03
    13 August 20212.2M+21.7%917K2.38
    30 July 20211.8M-4.2%1.1M1.65
    15 July 20211.9M-2.9%1.6M1.20
    30 June 20211.9M-44.7%2.1M1.00
    15 June 20213.5M+11.1%5.1M1.00
    28 May 20213.1M+3.2%973K3.22
    14 May 20213.0M-5.8%834K3.64
    30 April 20213.2M-0.6%920K3.50
    15 April 20213.2M-11.1%751K4.32
    31 March 20213.6M-8.0%1.0M3.50
    15 March 20214.0M+11.0%1.2M3.37
    26 February 20213.6M+4.4%1.0M3.57
    12 February 20213.4M-6.7%1.2M2.81
    29 January 20213.7M+0.7%847K4.33
    15 January 20213.6M-2.3%1.3M2.89
    31 December 20203.7M-3.1%1.1M3.28
    15 December 20203.8M-7.0%1.1M3.54
    30 November 20204.1M+11.6%1.2M3.44
    13 November 20203.7M-2.4%3.9M1.00
    30 October 20203.8M+26.0%1.1M3.34
    15 October 20203.0M+6.5%766K3.94
    30 September 20202.8M-23.3%1.2M2.32
    15 September 20203.7M+11.4%1.0M3.65
    31 August 20203.3M+39.1%1.0M3.31
    14 August 20202.4M-6.7%1.3M1.83
    31 July 20202.6M-8.2%940K2.72
    15 July 20202.8M-14.8%1.2M2.25
    30 June 20203.3M-29.9%2.5M1.28
    15 June 20204.7M+2.6%1.2M3.74
    29 May 20204.5M+1.3%1.0M4.53
    15 May 20204.5M+7.4%1.3M3.38
    30 April 20204.2M+9.4%2.1M1.98
    15 April 20203.8M+26.8%2.0M1.94
    31 March 20203.0M-15.6%2.4M1.25
    13 March 20203.6M+13.6%1.8M1.94
    28 February 20203.1M-7.0%1.7M1.89
    14 February 20203.4M+8.3%2.5M1.33
    31 January 20203.1M-13.4%1.5M2.03
    15 January 20203.6M-16.9%1.1M3.33
    31 December 20194.3M+3.2%1.3M3.35
    13 December 20194.2M-1.2%2.0M2.09
    29 November 20194.2M+27.0%1.8M2.30
    15 November 20193.3M-18.1%1.6M2.08
    31 October 20194.1M+23.6%4.1M1.00
    15 October 20193.3M-7.2%2.0M1.68
    30 September 20193.6M+2.4%1.6M2.18
    13 September 20193.5M-8.5%1.6M2.22
    30 August 20193.8M-3.3%1.1M3.44
    15 August 20193.9M-4.7%1.3M3.02
    31 July 20194.1M+8.7%1.4M3.03
    15 July 20193.8M+32.2%1.3M2.92
    28 June 20192.9M-27.6%2.1M1.35
    14 June 20194.0M+3.3%1.4M2.81
    31 May 20193.8M-1.8%1.7M2.29
    15 May 20193.9M-1.6%2.1M1.89
    30 April 20194.0M-3.4%2.4M1.66
    15 April 20194.1M+15.2%2.6M1.55
    29 March 20193.6M+24.3%5.6M1.00
    15 March 20192.9M-2.4%1.3M2.23
    28 February 20192.9M+7.2%901K3.26
    15 February 20192.7M-3.9%1.4M2.00
    31 January 20192.8M-20.6%1.3M2.12
    15 January 20193.6M-6.0%1.3M2.83
    31 December 20183.8M+4.3%1.7M2.21
    14 December 20183.7M+6.3%1.2M2.96
    30 November 20183.4M-6.8%1.1M3.10
    15 November 20183.7M+8.9%1.1M3.31
    31 October 20183.4M+5.8%1.8M1.90
    15 October 20183.2M+3.7%1.1M2.90
    28 September 20183.1M-2.0%1.5M2.12
    14 September 20183.2M-6.3%977K3.23
    31 August 20183.4M+0.5%840K4.00
    15 August 20183.3M-16.3%1.2M2.73
    31 July 20184.0M+2.4%2.5M1.59
    13 July 20183.9M-8.9%3.0M1.32
    29 June 20184.3M-9.3%1.5M2.77
    15 June 20184.7M+9.2%1.3M3.61
    31 May 20184.3M+12.2%1.6M2.71
    15 May 20183.9M+12.1%1.9M2.07
    30 April 20183.4M-1.6%2.0M1.76
    13 April 20183.5M+10.9%1.7M2.09
    29 March 20183.2M-10.6%1.9M1.70
    15 March 20183.5M+1.8%1.6M2.15
    28 February 20183.5M+2.0%2.1M1.65
    15 February 20183.4M+2.3%1.9M1.77
    31 January 20183.3M-2.3%1.4M2.34
    12 January 20183.4M-7.2%1.2M2.88
    29 December 20173.7M974K3.76
    Shares in millions (M) or thousands (K); hold the pointer on a figure for the exact count. FINRA publishes consolidated short interest twice a month, about ten days after each settlement.