It was a chilly evening in Mount Vernon, Wash., on March 10, when a group of singers met for choir practice at their church, just as they did most Tuesday nights.

The full choir consists of 122 singers, but only 61 made it that night, including one who had been fighting cold-like symptoms for a few days.

That person later tested positive for the coronavirus, and within two days of the practice, six more members of the choir had developed a fever. Ultimately, 53 members of the choir became ill with Covid-19, the disease caused by the virus, and two of them died.

The event, which was first reported by The Los Angeles Times in March, demonstrated how contagious and dangerous the coronavirus is, especially among older populations. The median age for those attending the practice that night was 69.

A study since then has showed that swift action by the members of the choir, including voluntary isolation, along with contact tracing by the Skagit County health department, helped contain the spread and prevent what could have been a much larger outbreak in that community, about an hour’s drive north of Seattle.

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Credit…Centers for Disease Control and Prevention

Although the virus spread quickly and thoroughly within the choir, it did not result in a significant increase in the infection rate of the community at large.

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“If they hadn’t initiated their own isolation and quarantine before we got involved, you can conceive of a situation where every one of those people would have infected another three people each,” said Dr. Howard Leibrand, the Skagit County health officer. “You would have had a huge change in our viral curve based on this one episode.”

As communities around the world begin to look at ways to ease restrictions on movement and gathering, as well as reopen their businesses and houses of worship, reports like this one raise clear warnings about the dangers posed when a large number of people gather indoors during an outbreak. That is especially the case for activities like singing that can spur transmission.

Dr. Leibrand and Dr. Lea Hamner, a communicable disease and epidemiology supervisor in Skagit County, conducted the investigation into the choir outbreak and were the authors of the report, which the Centers for Disease Control and Prevention released on Tuesday.

They labeled the choir practice a point-source exposure and said very few such events had been so clearly isolated as this one, making it a helpful study of how the virus spreads.

“It’s rare to have a group with a single common exposure,” Dr. Hamner said. “Here, we have a defined group, and they all had a similar exposure for a similar amount of time, so we are able to really understand transmission a little bit better with that kind of event.”

What made the choir practice so fertile for transmission was most likely the very act of singing, in which people projecting their voices at loud volume are prone to emit tiny droplets known as aerosols that can carry the virus. It is a phenomenon familiar to anyone who has sat close to the stage during a play or a musical performance.

“When you project your voice, you can project more virus,” Dr. Leibrand said. “So it seems like this would be a pretty good indicator we shouldn’t be going back to large groups singing in an enclosed space, i.e., church, because that would be the same sort of situation as this.”

Some people in the Skagit County choir may even have been what the report calls super-emitters — people who release more particles than others during speech.

As of Tuesday afternoon, the total number of cases in Skagit County was 406, according to the Washington Department of Public Health.

Pia MacDonald, an infectious disease epidemiologist for RTI International, who was not part of the study, said it was remarkable that the outbreak in the choir did not lead to more infections in the community at large, especially considering the cases were not confirmed until six days after the practice.

“You still have a number of days there where people could have been walking around in close contact with other people,” she said.

The practice took place from 6:30 to 9 p.m., and for much of it the choir members sat in a large room in assigned seats, correlating to how they would sit during a performance. The seats were packed together, six to 10 inches apart, far closer than the minimum six-foot recommendation by the C.D.C. during the pandemic.

Because only about half the members of the choir were present, many sat next to empty seats. But later they broke off into separate groups, and cookies and oranges were served during a break in the back of the room.

When the practice was over, several members helped to put the chairs away, and this contact with surfaces may also have contributed to transmission.

Weeks earlier, Washington reported its first coronavirus case, connected to a traveler from Wuhan, China, where the disease outbreak began, but awareness of the coronavirus in many parts of Washington and the United States was still in its early stages on March 10.

Dr. Hamner and Dr. Leibrand said that on March 15, the choir director sent an email to everyone in the choir reporting that six people had become ill. They said that from that point forward, the members of the choir were diligent about remaining isolated.

Three days later, they began working with the Skagit County health department, which initiated its investigation and began doing contact tracing. The doctors said the choir members were forthcoming and extremely helpful, not only with maintaining self-quarantine, but also with providing useful information that aided the investigation.

“They were the ideal group to do disease investigation with,” Dr. Hamner said. “I’ve done disease investigations for five years now, and that was a really nice relationship.”

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