CDC calls COVID-19 spread through Washington choir a ‘superspreader event’
SKAGIT COUNTY, Wash. — Early in the spread of COVID-19 in Washington, a choir in Skagit County held choir practice. One person was infected. Weeks later, the virus spread to nearly the entire group. Now, the CDC has completed its investigation into how the infection spread through the group.
According to the CDC, the Mount Vernon choir met for two and a half hours on March 10th. 61 members of the 122-member choir were in attendance. At the time, one member was showing cold-like symptoms.
By the 17th, several members of the choir had become ill and notified Skagit County Public Health. The health department obtained a list of the choir members and identified 53 cases, 33 confirmed and 20 probable. Three were eventually hospitalized; two of them died.
According to the CDC, members of the choir provided “several opportunities for droplet and fomite transmission” which included members sitting close to each other, sharing stacks and stacking chairs at the end of practice.
“The act of singing, itself, might have contributed to transmission through emissions of aerosols, which is affected by loudness of vocalization,” the investigation said. “Some people, known as superemitters, who release more aerosol particles during speech than do their peers, might have contributed to this and previously reported COVID-19 superspreading events.”
The investigation details the contact tracing epidemiologists did after the cases were identified. That included calling all 122 members of the group, focusing on who was at the practice on March 10th and also at practice the previous week.
Of those who attended the March 10th practice, 86% became ill with COVID-19. The CDC says choir members who attended that practice were 125.7 times more likely to become ill than those who did not.
The investigation details who was sitting where during that practice. It says the members were arranged in six rows of 20 chairs each, spaced 6-10 inches apart. Because not everyone attended, there were some empty seats. Attendees practiced together for 40 minutes, then split into smaller groups for an additional 50-minute practice. During a 15-minute break, members shared cookies and oranges. Then, they reconvened for a final 45-minute session. At the end, each member returned their chair to a stack and most left immediately. There was no known physical contact between members.
Among those attending that practice, the median age was 69 years old. Most developed symptoms one to 12 days after the practice. The first confirmed cases was identifed March 13th; the last person was tested March 26th.
The mean interval from illness onset to hospitalization was 12 days. “The intervals from onset to death were 14 to 15 days for the two patients who died,” the investigation said.
The health department collected signs and symptoms from the people who were infected. The most common symptoms were cough, fever, myalgia and headache. Several developed gastrointestinal symptoms later. Only one of them lost smell and taste. The most severe complications were viral pneumonia and respiratory failure.
The CDC also explored risk factors among those who got sick. The most common was age, with 75% of patients over 65. Most patients, 67%, did not report underlying medical conditions. All three hospitalized patients had two or more underlying conditions.
As part of the contact tracing investigation, the health district provided isolation and quarantine instructions by phone, email and postal mail. Contacts of those infected were traced and notified.
As part of its analysis, the CDC says this group provided a unique opportunity to understand this virus following a “likely-point exposure event.” Investigators say the “attack rate” in this group was higher than seen in other clusters and they consider the March 10th practice a “superspreader event.”
This outbreak, according to the CDC, indicates that the virus “might be highly transmissible in certain settings, including group singing events.” The investigation said it also shows the importance of physical distancing, avoiding group gatherings and crowded places and wearing cloth masks in public.
“The choir mitigated further spread by quickly communicating to its members and notifying [the health department] of a cluster of cases on March 18th.” When members were first contacted between March 18 and March 20th, the CDC says “nearly all persons who attended the practice reported they were already self-isolating or quarantining.”
You can read the full report and analysis at this link.
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