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A choir rehearsal in Washington state earlier this month is the likely catalyst for a coronavirus “super-spreading event” after dozens of members fell ill and two died from the infectious disease.

The Skagit Valley Chorale in Mount Vernon, located about an hour north of Seattle, held its weekly rehearsal at Mount Vernon Presbyterian Church on March 10, when there were no reported COVID-19 cases in the Skagit Valley area, the group said in a statement on its website last week.

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Choir leaders said they decided to make the practice optional considering the outbreak in other parts of Washington, and about 60 members – nearly half of the chorale – showed up.

Since the rehearsal, 45 members have become ill and 28 have tested positive for coronavirus, Skagit County Public Health told Q13 FOX on Monday. Two members in their 80s died.

At the time of the practice, there were no bans on large gatherings and no closures of any kind in the county, although the Skagit health department warned that those over 60 should avoid “large public gatherings."

Some members began feeling ill within the next several days, according to the group, and the number of sick only grew as more time passed.

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“This virus looks different on everyone," Carolynn Comstock, chorale co-president, told KOMO News. "Some people had a fever, some people didn’t. Some people had a cough, some people didn’t. Some people only had that loss of sense of smell and taste.”

Jamie Lloyd-Smith, a University of California, Los Angeles, infectious disease researcher told the Los Angeles Times that the gathering would be considered a “super-spreading event” because of the number of positive cases and participants with symptoms.

In interviews with the paper, eight members who attended the rehearsal said no one showed any signs of being ill or even coughed or sneezed. They said everyone avoided physical contact and brought their own sheet music.

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But one local health official believes it may have been the act of singing that led to the virus’ spread.

"I think singing with a wide-open mouth and projection of your voice can certainly cause droplet spread of the illness,” Dr. Howard Leibrand with Skagit Public Health told Q13 FOX. “We know that two people face to face in conversation can contaminate each other.”

For most people, the coronavirus causes mild or moderate symptoms, such as fever and cough. But for others, especially older adults and people with existing health problems, the virus can cause severe symptoms like pneumonia and even death.

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The virus has so far sickened at least 4,896 and killed 195 in Washington as of Tuesday, according to the state’s health department.