As many as 300,000 coronavirus cases across the United States can be traced 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 drug company Biogen, was one of the earliest examples in the pandemic of what epidemiologists call “superspreading events,” where a gathering of people leads to a huge number of infections. But new genetic data made publicly available in recent months by many states has allowed researchers for the first time to estimate the national scope of its astonishing ripple effect.
“It’s a cautionary tale,’’ said Bronwyn MacInnis, a genomic epidemiologist at the Broad Institute of Harvard and M.I.T. “When we hear these stories of clusters where 20 or 50 or 100 were affected, that does not account for what happens after.”
To track the spread, the researchers took advantage of a kind 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 that the conference had seeded tens of thousands of cases in the Boston area alone.
By March, the researchers had found, viruses bearing the same signature began to appear in the viral genomes taken from coronavirus patients in several other states. But by November, viruses containing the marker could be found in 29 states, linked to some 70,000 in Florida alone. And because the viral genome data linked to U.S. cases has grown by tenfold since June, the researchers are able to make a reliable national estimate. The conference, the study estimates, is responsible for 1.9 percent of all cases in the United States since the start of the pandemic.
A vast majority of introductions of the virus into a workplace or home or community fizzle, the researchers noted. But the study highlights how a local event with a mobile population can seed a national outbreak. Because the genetic fingerprint identified in the Biogen attendees existed previously in Europe, it was not possible to reliably estimate how many of the transmissions globally came from the Boston event, the researchers said.
Although the Biogen conference occurred at a time when the coronavirus was barely on the radar for most Americans, it might have important implications for the current pandemic moment. The first, eagerly awaited vaccines have been demonstrated to protect from severe Covid-19 symptoms, but it is not known whether they protect people from transmitting the virus.
“We risk having folks going around thinking ‘all is good,’” Dr. MacInnis said. “Our data reminds us what can happen when transmission is unchecked.’’
Los Angeles County could see “catastrophic suffering and death” in the coming weeks, public health officials warn, as the nation’s most populous county reported another record day of new coronavirus cases.
The 13,737 cases reported on Friday bring the county’s total to more than 500,000, as the county and California struggle to contain an explosion. California officials reported 37,124 cases on Friday, the highest one-day total of the pandemic.
“We’re on a very dangerous track to seeing unprecedented and catastrophic suffering and death here in L.A. County if we can’t stop the surge,” Barbara Ferrer, the county’s public health director, said at a news conference on Friday. “These numbers are overwhelming, and the grief that our community continues to experience can’t be comprehended.”
The number of people hospitalized for Covid-19 has sharply risen over the past month. In early November, fewer than 1,000 people were in hospitals for treatment. Today, there are more than 3,600, according to the latest figures from the county.
The steep rise in cases in California mirrors a national spike. Officials reported more than 236,000 new cases on Friday, yet another single-day case record.
During a news conference on Wednesday, Dr. Ferrer fought through tears as she talked about the state’s rising death toll.
“Over 8,000 people who were beloved members of their families are not coming back,” she said. “And their deaths are an incalculable loss to their friends and their families.”
Over the past month, the average number of daily deaths has increased more than 250 percent in Los Angeles County. In the past week, 357 people there have died and at least 71,079 people have contracted the virus.
“We’re in uncharted territory at this point,” Dr. Ferrer said. “We’re seeing daily numbers of cases and hospitalizations that we’ve not experienced and, frankly, did not anticipate.”
The Food and Drug Administration authorized Pfizer’s Covid-19 vaccine for emergency use on Friday, clearing the way for millions of highly vulnerable people to begin receiving the vaccine within days.
The authorization is a historic turning point in a pandemic that has taken more than 290,000 lives in the United States. With the decision, the United States becomes the sixth country — in addition to Britain, Bahrain, Canada, Saudi Arabia and Mexico — to clear the vaccine. Other authorizations, including by the European Union, are expected within weeks.
The F.D.A.’s decision followed an extraordinary sequence of events on Friday morning when the White House chief of staff, Mark Meadows, told the F.D.A. commissioner, Dr. Stephen Hahn, to consider looking for his next job if he didn’t get the emergency approval done on Friday, according to a senior administration official who spoke on condition of anonymity because he was not authorized to discuss the matter. Dr. Hahn then ordered vaccine regulators at the agency to do it by the end of the day.
The authorization set off a complicated coordination effort from Pfizer, private shipping companies, state and local health officials, the military, hospitals and pharmacy chains to get the first week’s batch of about three million doses to health care workers and nursing home residents as quickly as possible, all while keeping the vaccine at ultracold temperatures.
Pfizer has a deal with the U.S. government to supply 100 million doses of the vaccine by next March. Under that agreement, the shots will be free to the public.
Every state, along with six major cities, has submitted to the federal government a list of locations — mostly hospitals — where the Pfizer vaccine is to ship initially. In populous Florida, the first recipients will be five hospitals, in Jacksonville, Miami, Orlando, Tampa and Hollywood. In tiny, rural Vermont, only the University of Vermont Medical Center and a state warehouse will get supplies.
McKesson Corporation, a giant medical supplier, is sending kits of syringes, alcohol pads, face shields and other supplies to the same sites, where they will meet up with the vaccines that Pfizer is shipping in special boxes, packed with dry ice, designed to keep them at minus 94 degrees Fahrenheit.
The Pfizer packaging will include a device that tracks the location of the box, plus a thermal probe that will make sure the deep freeze is maintained throughout the journey from the company’s distribution sites in Michigan and Wisconsin.
The decision is a victory for Pfizer and its German partner BioNTech, which began working on the vaccine 11 months ago. Vaccines typically take years to develop. The companies’ late-stage clinical trial, which enrolled nearly 44,000 people, was found to be 95 percent effective.
The vaccine will be scarce at first. Pfizer had to scale back earlier estimates because of manufacturing setbacks, and has said it will be able to supply up to 25 million doses before the end of the year, and 100 million total vaccines by March.
Federal officials are initially holding back half of the supply so that they can give a booster shot to recipients three weeks after their first vaccination. Even though only about three million people will receive a vaccine in the first week, officials have held firm on their estimate that, between the Pfizer and Moderna vaccines, which each require two shots, they hope to give at least 20 million people their first dose of a vaccine by the end of the year.
An expert panel advising the F.D.A. on Thursday gave its approval of Pfizer’s vaccine for people 16 and older, and the agency was planning to release the formal authorization on Saturday. That timeline was accelerated by half a day after President Trump attacked Dr. Hahn for failing to authorize a vaccine more quickly. But the accelerated announcement was not expected to speed up the delivery of vaccines around the country.
With coronavirus cases surging, Gov. Andrew M. Cuomo of New York has shifted his strategy sharply away from tackling local clusters and toward protecting the state’s health system in a bid to avoid a return to the worst days of spring, when hospitals were stretched to the limit.
The virus statistic that had transfixed New Yorkers — the rate of tests that come back positive — is no longer the primary driver of state action, as it was when Mr. Cuomo sought to quash viral outbreaks in designated areas. That effort did not stem a rising tide of infections statewide, and the focus now is on hospital capacity.
Far from hastening a broad new round of business closings, the governor’s shift is likely to delay by weeks a potential return of the most stringent restrictions from earlier in the year. A rise in the number of hospitalizations follows an increase in positive cases, and the state has anticipated several steps hospitals can take to expand capacity before a shutdown is needed.
Still, there was one area where Mr. Cuomo was taking no chances. On Friday, he ordered a halt to indoor dining in New York City starting on Monday, saying that the ban was necessary to curb the surging outbreak. But the move prompted a backlash from the struggling restaurant industry, one of the city’s economic engines, with owners saying the governor had not proved that restaurants were a significant factor in spreading the virus.
After months of low positive test rates, New York is now in the same position as other states amid a worsening national outbreak: watching with increased concern as hospital beds fill.
More than 5,300 people were hospitalized across New York as of Thursday, a level not seen since May. The per capita rate of new cases in New York is better than it is in most states, but worse than in others, including Texas and Florida, although testing levels vary.
“If you extrapolate out at this rate of growth, you could be looking at the shutdown of New York City within a month,” Mr. Cuomo said in an interview on Friday.