A COVID-19 Superspreading Event at a Choir Practice, Skagit County, Washington, March 2020

This page summarizes a report from early in the COVID-19 pandemic, published in May 2020.

On March 17, 2020, a member of a 122-member choir in Skagit County, Washington, told the county health department that several members were sick. Three had tested positive for SARS-CoV-2, the virus that causes COVID-19, and another 25 had matching symptoms. The investigation that followed documented one of the early U.S. examples of a superspreading event.

Two practices

The choir met for a 2.5-hour practice every Tuesday evening through March 10. After the choir director e-mailed members on March 15 and 17 about the illnesses — the second message urging distancing and attention to symptoms — many isolated or quarantined themselves before the health department was even notified.

Health staff interviewed all 122 members by phone about the practices on March 3 and March 10, other gatherings and exposures, and symptoms. No one reported symptoms at the March 3 practice. But one person at the March 10 practice had had cold-like symptoms since March 7 and later tested positive; this person became the index patient.

The evidence pointed clearly to March 10. Onset for 92.5% of those who fell ill came between March 11 and 15, a median of 3 days after that practice. The odds of illness were 125.7 times higher for people who attended on March 10 than for those who did not, compared with 17.0 times for the March 3 practice. Of three members who attended only on March 10, two fell ill; of 21 who attended only on March 3, one did.

The evening of March 10

The practice ran from 6:30 to 9:00 p.m. in a large multipurpose room, with chairs in six rows of 20, spaced 6 to 10 inches apart. Half the members came, so some sat beside empty seats.

1. The whole group practiced for 40 minutes.
2. It split into two groups for another 50 minutes, one moving to a smaller room; people in both rooms now sat next to one another.
3. During a 15-minute break, cookies and oranges were set out at the back, though many did not eat.
4. Everyone returned to their original seats for a final 45 minutes.
5. At the end, members stacked their own chairs, crowding around the chair racks.

No one reported physical contact, and most people left right away.

The outbreak

Of the 60 attendees besides the index patient, 52 (86.7%) became ill: 32 confirmed by testing and 20 probable. That is a secondary attack rate of 53.3% counting only confirmed cases and 86.7% counting all. The attendees' median age was 69 and 84% were women, matching those who got sick. Symptoms began 1 to 12 days after the practice.

Three patients were hospitalized, an average of 12 days after falling ill, and two died, 14 and 15 days after their illness began. Among confirmed cases, the most common symptoms were cough, fever, muscle aches and headache; some developed diarrhea, nausea or abdominal pain, and one lost only the senses of smell and taste. The most serious complications were viral pneumonia and severe respiratory failure with low blood oxygen.

Why it spread so widely

The practice offered many chances for droplet and surface transmission: people sitting close together, sharing snacks and stacking chairs. Singing itself may have spread the virus through aerosols, whose release depends on how loudly people vocalize, and some people — "superemitters" — release more aerosol particles when they speak than others, which may help explain this and other superspreading events.

Lessons

The outbreak demonstrated how easily SARS-CoV-2 spreads. The authors stressed physical distancing, including avoiding large gatherings, and community awareness that encourages people with symptoms, and their contacts, to isolate or quarantine.

Sources

• Lea Hamner, Polly Dubbel, Ian Capron, Andy Ross, Amber Jordan, Jaxon Lee, Joanne Lynn, Amelia Ball, Simranjit Narwal, Sam Russell, Dale Patrick and Howard Leibrand, "High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice — Skagit County, Washington, March 2020," Morbidity and Mortality Weekly Report, CDC: https://www.cdc.gov/mmwr/volumes/69/wr/mm6919e6.htm
• The report's figures are not reproduced.