Superspreading Boston biotech convention in February is linked to 1.9 p.c of all U.S. instances.

Up to 300,000 coronavirus cases in the United States can be traced back to a two-day conference in Boston attended by 175 biotech executives in February, according to a study published Thursday in the journal Science.

The conference, convened by the pharmaceutical company Biogen, was one of the earliest examples in the pandemic of what epidemiologists refer to as “widespread events” where a congregation of people leads to large numbers of infections. Thanks to new genetic data made publicly available by many states in the past few months, researchers have for the first time been able to estimate the national extent of their amazing ripple.

“It’s a cautionary story,” said Bronwyn MacInnis, a genomic epidemiologist at the Broad Institute of Harvard and MIT. “When we hear these stories of clusters where 20, 50 or 100 were affected, it doesn’t explain what happens after that.”

To track the spread, the researchers used a type of genetic fingerprint that they identified in samples of the virus taken from 28 people who had attended the meeting. An earlier version of the paper, published online in June, suggested the conference had sparked tens of thousands of cases in the Boston area alone.

By March, the researchers had found that viruses with the same signature appeared in the viral genomes of coronavirus patients in several other states. Viruses containing the marker had been found in 29 states by November, including around 70,000 in Florida alone. And because the viral genome data associated with US cases has increased ten-fold since June, researchers can make a reliable national estimate. The study estimates that the conference has been responsible for 1.9 percent of all cases in the United States since the pandemic began.

The vast majority of introductions of the virus are at work, at home, or in the community, the researchers said. However, the study shows how a local event involving a mobile population can trigger a national outbreak. Because the genetic fingerprint identified on the Biogen attendees previously existed in Europe, it was not possible to reliably estimate how many of the transmissions worldwide came from the Boston event, the researchers said.

Although the Biogen conference came at a time when coronavirus was barely on the radar for most Americans, it could have important implications for the current pandemic moment. The first highly anticipated vaccines have been shown to protect against severe Covid-19 symptoms. However, it is not known if they protect people from transmitting the virus.

“We risk people walking around thinking everything is fine,” said Dr. MacInnis. “Our data reminds us of what can happen when transmission is disabled. ”