By David Williams, CNN
(CNN) — Public health officials studying the COVID-19 outbreak among members of a Washington choir found numerous ways the virus could have spread, according to a report published by the Centers for Disease Control and Prevention.
Authorities interviewed all 122 members of the Skagit Valley Chorale, which met every Tuesday for 2½ hours until the outbreak. They focused on a March 10 rehearsal attended by 61 members in Mount Vernon, Washington, 50 miles north of Seattle.
The report said 53 people were sickened and two died. Thirty-three of the cases were confirmed as coronavirus through testing, and the other 20 have been deemed probable infections.
Of those sickened, 52 had attended the March 10 rehearsal, and the other one had attended the March 3 rehearsal.
The first case identified by health authorities was a person who attended both rehearsals and reported having cold-like symptoms starting March 7.
Skagit County officials issued social distancing recommendations on March 10, but most people were probably not yet aware of them on that evening.
No one reported physical contact between the attendees at the practice, but they sat close together. The chairs were 6 to 10 inches apart, with empty seats between some of the members.
The choir broke into two groups for part of the practice. Members moved closer together for that 45-minute session, they said.
“The act of singing, itself, might have contributed to transmission through emission of aerosols, which is affected by loudness of vocalization,” the report said. The report also said that some people, known as superemitters, release more aerosol particles during speech than their peers.
There was a 15-minute snack break with cookies and oranges.
Each member carried their own chair before and after the rehearsal and congregated around a chair rack, the report said.
“This underscores the importance of physical distancing, including maintaining at least 6 feet between persons, avoiding group gatherings and crowded places, and wearing cloth face coverings in public settings where other social distancing measures are difficult to maintain during this pandemic,” the report said.
Those who were infected developed symptoms one to 12 days after the March 10 practice, the report said.
Most of the singers were women (84%) and their median age was 69, the report said. Thirty-two percent reported having underlying health conditions.
The choir emailed members March 15 to let them know that some people had gotten sick. Many had self-quarantined by the time health officials contacted them, and the report said this may have mitigated further spread.
The study was conducted by Skagit County Public Heath officials and published in the CDC’s Morbidity and Mortality Weekly Report.
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