As the SARS-CoV-2/COVID-19 pandemic continues across the globe, researchers and epidemiologists are working tirelessly to understand clusters that shed new insight into transmission dynamics.
From healthcare exposures to household contacts, there is a varying degree of risk based upon the environment and the interaction.
Similar to superspreader events of SARS-CoV and MERS-CoV, a cluster in Skagit County, Washington, has resulted in a high attack rate of cases. In the latest Morbidity and Mortality Weekly Report
(MMWR) from the Centers for Disease Control and Prevention, a review of this particular choir cluster reveals how easy transmission can occur.
Mid-March of this year, a member of a choir notified Skagit County Public Health that several members of their 122-person choir had become ill and three had tested positive for SARS-CoV-2. Beyond those initial cases, another 25 people were also symptomatic, which triggered a public health investigation the following day.
While the choir was 122 people, the March 10 practice involved 61 people, of which one was symptomatic. As no members of the choir reported symptoms from the March 3 choir practice, the March 10 one was utilized for contact tracing.
From this line list, investigators found 53 cases, included 33 confirmed and 20 probable cases, leading to an attack rate of 53.3% among those confirmed and an overall attack rate of 86.7% among all cases.
Of those 52 cases, 5.7% were hospitalized and 3.7% died. When reviewing the activities of the choir practice, the investigation shed light on how close, prolonged contact is a key to SARS-CoV-2 transmission.
The March 10 practice was 2.5 hours long and not only included close contact and singing, which helped droplet transmission through loud vocalization, but also several opportunities for indirect transmission through shared item like snacks.
The authors noted that the choir, which included 122 members, met for a 2.5-hour practice every Tuesday evening through March 10.
"On March 15, the choir director e-mailed the group members to inform them that on March 11 or 12, at least 6 members had developed fever and that 2 members had been tested for SARS-CoV-2 and were awaiting results,” authors wrote.
Public health investigators contacted all 122 choir members by phone during the initial investigation and performed follow-up calls the second week in April to identify and additional transmission.
What’s interesting is that 78 members attended the March 3 practice, while only 61 attended the March 10, and yet such a significant exposure occurred with the later. Of those attending the March 3 practice, 51 (65.4%) became sick as all but one also attended the March 10 practice.
Investigators emphasized that the long rehearsal, close quarters, and nature of the interaction were a prime environment for transmission. Moreover, the median age of those 61 choir members was 69 years, and the majority of symptoms were cough, fever, and myalgia.
In response, public health officials ensured the March 10 attendees were isolated and quarantined and had access to the information necessary to be safe and report symptoms. Lastly, the authors noted that the attendees had multiple opportunities for SARS-CoV-2 transmission by droplets, close contact, or fomite.
"Aerosol emission during speech has been correlated with loudness of vocalization, and certain persons, who release an order of magnitude more particles than their peers, have been referred to as superemitters and have been hypothesized to contribute to superspeading events," they wrote.
Singing while 6-10 inches from one another could be constituted as "intense and prolonged exposure" risk, authors noted.
Though this particular cluster of cases is fascinating and while the investigation had its own limitations, like unknown seating assignments and some probable cases, it sheds light on how certain environments and interactions can increase the risk for transmission.
Furthermore, it reinforces the importance of avoiding groups larger than 10 during the pandemic.