COVID-19: High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice – Skagit County, Washington, March 2020

COVID-19: High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice – Skagit County, Washington, March 2020. Spencer EA, Heneghan C.

Published on July 2, 2020

Reference Hamner L, Dubbel P, Capron I, et al. High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice - Skagit County, Washington, March 2020. MMWR Morb Mortal Wkly Rep. 2020;69(19):606-610. Published 2020 May 15.  2020
Study type
Country USA
Setting Choir practices
Funding Details All authors report receipt of funding through Public Health Emergency Preparedness grant from the Washington State Department of Health during the conduct of the study.
Transmission mode Person to person, Close contact, Aerosol, Fomite

Bottom Line

This study of choir practice attendees suggests that transmission of COVID-19 was facilitated by close proximity and physical contact and may have been augmented by the act of singing.

Evidence Summary

A 2.5-hour choir practice was attended by 61 persons, including a symptomatic index patient 

Subsequently, 32 confirmed and 20 probable secondary COVID-19 cases were identified (attack rate for confirmed cases = 53%, the attack rate for probable cases = 87%) Three patients were hospitalized, and two died. 

There were several opportunities for droplet and fomite transmission, including members sitting close to one another, sharing snacks, and stacking chairs at the end of the practice. The act of singing might have contributed to the transmission through the emission of aerosols, which is affected by the loudness of vocalization.

What did they do?

Skagit County Public Health (SCPH) in Washington State, USA was informed on 17th March 2020 that several members of Skagit County choir had become ill: 3 had tested positive for SARS-CoV-2; 25 more had symptoms consistent with COVID-19 infection. 

SCPH obtained the choir’s list of 122 members and began an investigation on 18th March. All 122 members were interviewed by telephone either during the initial investigation of the cluster (March 18 to 20; 115 members) or a follow-up interview (April 7 to 10; 117); most persons participated in both interviews

61 individuals attended choir practice on 10th March; at this time one person was known to be symptomatic. Among the 61, subsequently, 53 cases were identified, including 33 confirmed and 20 probable cases (secondary attack rates of 53.3% among confirmed cases and 86.7% among all cases). 

Three of the 53 people who became ill were hospitalized (5.7%), and two died (3.7%). 

Study reliability

This is a study of the characteristics of one particular setting. 

The seating chart at choir was not reported because of concerns about patient privacy. However, with attack rates of 53.3% and 86.7% among confirmed and all cases, respectively, and one hour of the practice occurring outside of the seating arrangement, the seating chart may not add substantive additional information. 

The 19 choir members classified as being probable cases did not seek testing to confirm their illness. One person classified as having probable COVID-19 did seek testing 10 days after symptom onset and received a negative test result. It is possible that persons designated as having probable cases had another illness.

Clearly defined setting Demographic characteristics described Follow-up length was sufficient Transmission outcomes assessed Main biases are taken into consideration
Yes Yes Yes Yes No

What else should I consider?

There are several interactions that preclude a single cause to be assigned to choir practices as a cause of superspreading.

About the authors

Carl Heneghan

Carl is Professor of EBM & Director of CEBM at the University of Oxford. He is also a GP and tweets @carlheneghan. He has an active interest in discovering the truth behind health research findings

Elizabeth Spencer

Dr Elizabeth Spencer; MMedSci, PhD. Epidemiologist, Nuffield Department for Primary Care Health Sciences, University of Oxford.