In Washington state, the Skagit Valley Chorale became infected with the coronavirus.

On March 10, 2020, a three-hour choir rehearsal was held at a mount Vernon Church. There were no large-scale COVID-19 infections in Skagit County, so mass events were still allowed. However, the Skagit County Health Department recommended that seniors and others at risk avoid large public gatherings. Choir members were asked not to come to rehearsals if they had cold symptoms, even if they thought it was just a seasonal allergy.

The Skagit Valley chorale has 122 participants: singers, conductor, and accompanist. But due to fears of Contracting the coronavirus, only 61 participants came to the rehearsal. During the rehearsal, precautions were taken: hand sanitizer was used, and hugs and handshakes were forbidden. To ensure social distance, 120 chairs were set up for the singers, as usual. The singers sat in their regular seats, but their space increased due to the smaller number of participants. The singers’ lateral distance was 0.75 m, and the distance between the rows was 1.4 m.

However, one of the participants in the rehearsal had symptoms of a cold. Later it turned out that this person is sick with COVID-19. Despite the precautions taken, 53 members of the Chorale became infected with the coronavirus.

Members of the Chorale began to get sick between March 11 and 22. The incubation period of SARS-CoV-2 coronavirus is from 1 to 14 days. The incidence was low before the choir rehearsal in Skagit County, and then there was a spike, confirming that the mass infection occurred at the rehearsal on March 10. If the disease had happened earlier, given the incubation period, 80% of the participants would have already had symptoms by March 10. They are unlikely for asymptomatic cases, since the sick choir members are mostly elderly, aged 65 and older.

Possible ways of transmitting coronavirus during the Skagit Valley Chorale rehearsal

There are three main ways of transmitting COVID-19:

  • direct contact (such as a handshake) or indirect contact with infected objects followed by touching the eyes, nostrils, or mouth;
  • large drops that are released from the nose or mouth of an infected person when sneezing or coughing and enter the eyes, nostrils, or mouth of a healthy person;
  • exhaled respiratory aerosols remaining in the air for several minutes to hours and can infect when inhaled.

There was no direct contact between the choir members. Some participants helped arrange chairs, snacked on tangerines, and visited the toilet. But among the sick were those who did not do it. Therefore, it is unlikely that the coronavirus was transmitted by contact.

A participant with coronavirus symptoms was close to only a small percentage of the singers. There was no one 3 meters in front – which is the distance that large drops can reach when sneezing and coughing – and within 1 meter to the right and left and 2 meters behind. Ventilation in the hall was low, so infected drops could not fly back.

During the rehearsal, there was a 10-minute break, during which the choir members talked to each other in groups of 3-4 people. But the infected participant had minimal communication with others. Many people came to the rehearsal at the very beginning and left immediately after the end. Considering that it takes about 15 minutes to transmit SARS-CoV-2, it is unlikely that the infected participant had time to cough or sneeze next to all 53 patients.

Chorus spokesperson Carolyn Comstock confirms this: ” The group is seriously about the music, not overly social. Our director keeps strict control over the time we have to maximize the learning.”

These facts rule out the dripping mode of transmission of coronavirus.

There is a third way – through an infected respiratory aerosol.

Poor ventilation caused the outbreak of the coronavirus in choir practice

A relatively new commercial oven heats the hall where the chorale rehearsal took place with forced air supply. Before the rehearsal, the heating system was turned on at 20°C. It was 7°C outside, and the heating was turned off shortly after the rehearsal started because, with a large number of people, the human body emits enough heat to maintain a comfortable indoor temperature. The front doors were closed throughout the rehearsal.

According to the American standard “Ventilation for acceptable indoor air quality,” for a hall with a volume of 810 m3, which holds 61 people, you need from 2.5 to 10 air exchanges per hour. At the rehearsal on March 10, this standard was not met.

Laboratory studies have shown that the SARS-CoV-2 virus can remain contagious while in the air for a long time. The viral load of the exhaled aerosol changes during the disease and reaches a maximum when symptoms appear. The rate of aerosol release during singing increases with the volume of the voice.

Conditions that contribute to COVID-19 outbreaks include large indoor gatherings and poor ventilation. A Japanese study based on contact tracking found that the probability of transmitting COVID-19 indoors is 19 times higher than outdoors.

During the rehearsal, all the conditions were present for the virus to spread: high attendance, long duration, loud vocalization, and poor ventilation. These facts confirm that the inhalation of infected respiratory aerosol was the leading mode of transmission of SARS-CoV-2.

Conclusions

During the coronavirus pandemic, crowded indoor gatherings should be suspended. Risks are reduced if fewer people attend meetings, the meeting duration gets shorter, and all participants wear masks.

Since singing releases many respiratory aerosols that can be infected, group singing indoors is best avoided.

To prevent coronavirus spread in the premises, it is necessary to provide enhanced ventilation, mechanical filtration, and bactericidal ultraviolet disinfection.

All these measures will help limit the extent of infection and slow the spread of the pandemic.

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