BOSTON (NYTIMES) - On the first Monday in March, Mr Michel Vounatsos, chief executive of the drug company Biogen, appeared in good spirits. The company's new Alzheimer's drug was showing promise after years of setbacks. Revenues had never been higher.

On stage at an elite health care conference in Boston, Mr Vounatsos touted the drug's "remarkable journey". Asked if the coronavirus that was ravaging China would disrupt supply chains and upend the company's big plans, he said no.

"So far, so good," he said.

But even as he spoke, the virus was already silently spreading among Biogen's senior executives, who did not know they had been infected days earlier at the company's annual leadership meeting.

Biogen employees, most feeling healthy, boarded planes full of passengers. They drove home to their families. And they carried the virus to at least six US states, the District of Columbia and three countries, outstripping the ability of local public health officials to trace the spread.

The Biogen meeting was one of the earliest examples in the United States of what epidemiologists call "superspreading events" of Covid-19, where a small gathering of people leads to a huge number of infections.

Unlike the most infamous clusters of cases stemming from a nursing home outside Seattle or a 40th birthday party in Connecticut, the Biogen cluster happened at a meeting of top healthcare professionals whose job it was to fight disease, not spread it.

"The smartest people in healthcare and drug development - and they were completely oblivious to the biggest thing that was about to shatter their world," said Mr John Carroll, editor of Endpoints News, which covers the biotech industry.

The official count of those sickened - 99, including employees and their contacts, according to the Massachusetts Department of Public Health - includes only those who live in that state. The true number across the US is certainly higher.

The first two cases in Indiana were Biogen executives. So was the first known case in Tennessee, and six of the earliest cases in North Carolina.

All the people outside Massachusetts that The New York Times connected to the cluster have recovered. But it's impossible to say for certain whether anyone became gravely ill or died from the spread-out of the conference.

With hindsight, many people criticised Biogen's decision to continue with its leadership meeting in late February, which was attended by vice-presidents from European countries already hit by the virus. Others in the industry fault Biogen for being too tight-lipped about the outbreak.

Related Story Wild weather set to tear across US, adding to coronavirus woes

Related Story Trump likes coronavirus briefings, some advisers worry he likes them too much

The company has defended its handling of the leadership meeting and its aftermath, saying it made the best decisions it could with the information available at the time.

"For a company whose mission is to save lives, it was very difficult to see our colleagues and community directly affected by this disease," Mr Vounatsos said in his first public comments about what happened at Biogen. "We would never have knowingly put anyone at risk."

Founded in 1978 and based near Boston, Biogen helped pioneer the biotechnology industry, specialising in multiple sclerosis drugs. The company is best known now for its work on a promising treatment for Alzheimer's.

By the time of Biogen's annual leadership meeting on Feb 26 and 27, spirits were high. So was the pressure to deliver.

Although some other companies cancelled international meetings around that time, Biogen never discussed doing so. The outbreak was raging in China but had not yet been declared a worldwide pandemic.

As of Feb 21, the Friday before the meeting, the United States had only 30 confirmed cases, according to data compiled by The Times. Biogen executives in Germany, Switzerland and Italy - where there were just 20 known cases - packed their bags.

On the first night, about 175 executives gathered for a buffet dinner and cocktails at the Marriott Long Wharf overlooking Boston Harbour. Colleagues who hadn't seen one another in a year shook hands and vied for face time with bosses. Europeans gave customary kisses on both cheeks.

"It's unfortunately the perfect breeding ground for a virus," said one former vice-president, who spoke on condition of anonymity because of his ties to Biogen.

Two days later, the senior executives returned to their offices. One drove to a manufacturing centre in North Carolina. Others flew back to Europe.

Mr Peter Bergethon, the head of digital and quantitative medicine at Biogen, went home to his wife, an infectious-disease doctor.

A Biogen vice-president in the Alzheimer's franchise and her husband attended a party the following Saturday night at a friend's home in Princeton, New Jersey, with about 45 other people.

They celebrated a holiday in the Greek Orthodox calendar, the end of the Carnival season, with special sweets and traditional dances that involved holding hands in a circle. Although celebrations in Greece had been cancelled, the party in New Jersey went forward, since White House officials had just pronounced the virus in the US to be under control.

That night, Ms Allana Taranto, a photographer who covered the leadership meeting for Biogen, celebrated her 42nd birthday with her boyfriend and another couple.

Over that weekend, though, some people in the company started feeling sick.

Ms Jie Li, a 37-year-old biostatistician who worked on the Alzheimer's drug team, had chills, a cough and aches. She was too junior to attend the company's leadership conference, but her boss went, and showed up at the office afterward.

On March 2, the following Monday, the company's chief medical officer sent an e-mail informing everyone who attended the leadership meeting that some people had fallen ill and telling them to contact a healthcare provider if they felt sick.

"We moved quickly," Biogen spokeman David Caouette said.

Still, that same day, the company's four top executives attended a huge healthcare conference hosted by the investment firm Cowen. At another Marriott in Boston, they held meetings in hotel rooms with potential investors. Another attendee who met some of the same investors said he heard that members of the Biogen team looked sick.

At the conference, concern about the coronavirus mounted as word spread that some companies, including Vertex and Seattle Genetics, had cancelled their appearances. By Tuesday, the second day of the conference, many attendees had stopped shaking hands.

The next day, confirmation of the worst arrived. Two Biogen executives who had returned home to Germany and Switzerland, where tests were more widely available, had tested positive.

On Thursday, the company held a call with its staff and shared the news. All office-based employees were directed to work from home.

Yet on that same day, a Biogen executive visited the Washington office of Pharmaceutical Research and Manufacturers of America, or PhRMA, the industry's top lobbying group. Soon after, that executive tested positive, prompting the group to close its headquarters for deep cleaning.

The next few weeks turned into a blur of Biogen employees leaving casseroles on one another's doorsteps and trading news about who had fallen ill.

Mr Bergethon infected his wife, the infectious-disease specialist. While their symptoms were manageable, the scariest part was the uncertainty, Mr Bergethon recalled recently at a virtual event hosted by the University of Rochester.

"We didn't know we were going to recover," he said. "We didn't know what was coming next."

Ms Taranto, the photographer who had been at Biogen's leadership conference, unknowingly gave the illness to a friend at her birthday dinner. She had felt healthy at the time.

Of the four dozen people who attended the party in New Jersey, at least 15 later tested positive, according to public health authorities.

A Biogen executive, Mr Chris Baumgartner, became the first Covid-19 case in Tennessee. "I was patient zero," he wrote on Facebook.

He added: "Imagine having to confront a virus so feared, it now has the entire world on the brink of mass hysteria."