An international meeting of Biogen leaders at a Boston hotel in February led to roughly 20,000 cases of COVID-19 in four Massachusetts counties by early May, far more than the 99 previously identified, according to three scientists involved in a new study.

After examining nearly all the confirmed early cases of the illness in the area by changes in the genetic makeup of coronaviruses as they pass from one person to another, the researchers were able to assess the broader impact of the “super-spreading event” at the Marriott Long Wharf hotel.

What makes the estimate all the more startling is that this version of the virus was one of more than 80 that invaded the state between late January and early May, the researchers found. But it sparked a viral forest fire.

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