Who owns BIIB (BIIB)

How the tape is positioned

240 tracked funds added or opened BIIB while 168 trimmed or exited it between 2025-Q4 and 2026-Q1 — the tape is split. When large holders disagree on a name, the funds moving first are sometimes ahead of the story.

A prompt to read the filings, not a verdict. 13F is a quarter-end snapshot disclosed up to 45 days late; a fund adds or exits for mandate, benchmark, and timing reasons that have nothing to do with a view on the stock. This is a starting question, not a signal to trade.

Conviction spread

BIIB’s largest tracked holder by dollar value, VANGUARD GROUP INC, sizes it at 0.0% of its own reported 13F book, while the median holder sizes it at 0.1% of theirs — a 0.0pp gap the other way — the median holder sizes BIIB more heavily than its largest holder by dollar value does.

Book-share is each fund’s reported value in BIIB divided by its whole reported 13F book, so it is comparable across funds of different sizes and is unaffected by stock splits. A higher share means a fund has committed more of its book to the name — a starting question about conviction, not a verdict or a signal to trade.

Institutional & insider positioning

Between 2025-Q4 and 2026-Q1, across 998 tracked filers holding BIIB, 527 increased or opened the position and 425 reduced or closed it; over the same quarter, open-market insider Form 4s were net purchases.

This fuses two public signals for the same quarter — how tracked institutions changed their positions and whether insiders bought or sold in the open market. It describes what was filed; it is a starting question, not a verdict or a signal to trade.

Ownership over time

Tracked institutional holder count and total shares held by filing quarter (independent scales). The curated-era tail and any mid-ingest latest quarter are handled honestly — a partial quarter is not drawn as a collapse.