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Choir practice that left 87% members infected is proof one gathering can be a coronavirus 'super spreader': CDC

Among 61 people who attended a March 10 choir practice, 53 were infected with COVID-19 and two of them died
(Getty Images)
(Getty Images)

Even one social gathering can quickly spread the new coronavirus, shows an analysis of a choir practice, where one person who did not show any symptoms led to 87% of others developing Covid-19 in Washington, US.

Following a 2.5-hour choir practice attended by 61 people, including one asymptomatic index patient, 32 confirmed and 20 probable secondary COVID-19 cases occurred. Three of the 53 people who became ill were hospitalized (5.7%) and two died (3.7%), according to an analysis by the US Centers for Disease Control and Prevention (CDC).

The attack rate in this group (53.3% and 86.7% among confirmed cases and all cases, respectively) was higher than that seen in other clusters, and the March 10 practice could be considered a super-spreading event, say experts. According to the researchers, the transmission was likely facilitated by close proximity (within 6 feet) during practice and augmented by the act of singing. 


The investigation

On March 17, a member of a Skagit County (Washington) choir informed Skagit County Public Health (SCPH) that several members of the 122-member choir had become ill. Three people, two from Skagit County and one from another area, tested positive for SARS-CoV-2, the virus that causes Covid-19. Another 25 people had compatible symptoms.

The SCPH obtained the choir’s member list and began an investigation on March 18. None of the choir members reported having any symptoms at the March 3 practice. One person at the March 10 practice had cold-like symptoms beginning March 7. This person, who had also attended the March 3 practice, had a positive laboratory result for SARS-CoV-2 by reverse transcription-polymerase chain reaction (RT-PCR) testing. In total, 78 members attended the March 3 practice, and 61 attended the March 10 practice.

Overall, 51 (65.4%) of the March 3 practice attendees became ill. All but one of these people also attended the March 10 practice. Among 60 attendees at the March 10 practice (excluding the patient who became ill on March 7, who also attended), 52 (86.7%) choir members subsequently became ill. Some members exclusively attended one practice. Among 21 members who only attended March 3 practice, one became ill and was not tested (4.8%), and among the three members who only attended March 10, two became ill (66.7%) with one Covid-19 case being laboratory-confirmed.

Number of choir members with and without Covid-19–compatible symptoms, and members’ choir practice attendance at Skagit County, Washington, March 3 and 10 (CDC)

Among people with confirmed infections, the most common signs and symptoms reported at illness onset and at any time during illness were cough (54.5% and 90.9%, respectively), fever (45.5%, 75.8%), myalgia (27.3%, 75.0%) and headache (21.2%, 60.6%). Several patients later developed gastrointestinal symptoms, including diarrhea (18.8%), nausea (9.4%), and abdominal cramps or pain (6.3%). One person experienced only a loss of smell and taste. The most severe complications reported were viral pneumonia (18.2%) and severe hypoxemic respiratory failure (9.1%).

Among the recognized risk factors for severe illness, the most common was age, with 75.5% of patients aged 65 years or more. Most patients (67.9%) did not report any underlying medical conditions, 9.4% had one underlying medical condition and 22.6% had two or more underlying medical conditions. All three hospitalized patients had two or more underlying medical conditions.

The researchers say that the singing practice provided 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 itself might have contributed to the transmission through the emission of aerosols, which is affected by the loudness of vocalization, say experts.

The team explains that aerosol emission during speech has been correlated with the loudness of vocalization, and certain people, who release more particles than their peers, have been referred to as ‘super-emitters’ and have been hypothesized to contribute to super-speeding events. Members had an intense and prolonged exposure, singing while sitting 6-10 inches from one another, possibly emitting aerosols, they added. 

"Certain persons, known as super-emitters, who release more aerosol particles during speech than do their peers, might have contributed to this and previously reported COVID-19 super-spreading events. These data demonstrate the high transmissibility of SARS-CoV-2 and the possibility of super-emitters contributing to broad transmission in certain unique activities and circumstances," the research team says in their findings. 

Certain persons, known as super-emitters, who release more aerosol particles during speech than their peers, might have contributed to the transmission (Getty Images)

The researchers say while multiple reports have documented events involving super-spreading of Covid-19, but few have documented a community-based point-source exposure. This cluster of 52 secondary cases of Covid-19 presents a unique opportunity for understanding SARS-CoV-2 transmission following a likely point-source exposure event, say experts. According to their analysis, people infected with SARS-CoV-2 are most infectious from two days before through seven days after symptom onset. The mean interval from illness onset to hospitalization was 12 days. The intervals from onset to death were 14 and 15 days for the two patients who died, shows analysis. 

"The index patient developed symptoms on March 7, which could have placed the patient within this infectious period during the March 10 practice. Choir members who developed symptoms on March 11 (three) and March 12 (seven) attended both the March 3 and March 10 practices and thus could have been infected earlier and might have been infectious in the 2 days preceding symptom onset (that is, as early as March 9). The median incubation period of Covid-19 is estimated to be 5.1 days. The median interval from exposure during the March 10 practice to the onset of illness was 3 days, indicating a more rapid onset," says the report. 

Based on their findings, the team says that this outbreak of Covid-19 with a high secondary attack rate indicates that SARS-CoV-2 might be highly transmissible in certain settings, including group singing events. This underscores the importance of physical distancing, including maintaining at least 6 feet between persons to reduce transmission, say experts. The researchers recommend avoiding group gatherings, crowded places and face-to-face contact with others, as well as wearing cloth face coverings in public settings where other social distancing measures are difficult to maintain during the coronavirus pandemic.