Maggie Fox of CNN reports on the downstream COVID-19 infections resulting from the Biogen conference last February: “Boston biotech conference led to 245,000 Covid-19 cases across US, genetic fingerprinting shows”.
One was especially bad. A virus carrying one mutation -- a small genetic change they've flagged as C2416T -- was apparently carried to the conference by a single person, and ended up infecting 245,000 people. A subset of the viral strain with a mutation known as G26233T ended up in 88,000 of these cases.
There were only around 200 attendees of the conference.