(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 2.5 hours before the outbreak. They focused on two rehearsals held March 3 and March 10 in Mount Vernon, Washington.
The report said 53 people were sickened and two died -- and all but one attended both rehearsals. The report said Thirty-three cases were confirmed, the report said, and 20 people had probable infections.
There were 61 people at the March 10 rehearsal, including one member who reported having had cold-like symptoms. That person tested positive for Covid-19 and was the first case identified by health authorities, according to the report.
That person attended both practices but didn't start showing symptoms until March 7.
County officials issued social distancing recommendations on March 10, but most people were probably not yet aware of them, according to a statement from Skagit County Public Health.
No one reported physical contact between the attendees at the practices, but they sat close together. The report said the chairs were 6-10 inches apart, but there were 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 also a 15-minute snack break with cookies and oranges. Members moved the chairs 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.
Attendees developed symptoms between one day 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. Only 32% 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.