High SARS-CoV-2 attack rates following exposure during five singing events in the Netherlands, September-October 2020

BackgroundPrevious reports suggest SARS-CoV-2 transmission risk increases during singing events. From September-October 2020, several clusters of COVID-19 cases among singing events were reported across the Netherlands. Our aim was to investigate whether singing increased SARS-CoV-2 transmission risk during these events. MethodsData from 5 events were retrospectively collected from spokespersons and singing group members via questionnaires. Information was consolidated with the National Notifiable Diseases Surveillance System. Specimens were requested for sequencing for point source and cluster assessment. We described outbreaks in terms of person, place and time and depicted potential SARS-CoV-2 transmission routes. A previously published model (AirCoV2) was used to estimate mean illness risk of 1 person through airborne transmission under various scenarios. ResultsEvents included 9-21 persons (mean: 16), aged 20-89 years (median: 62). Response rates ranged 58-100%. Attack rates were 53-74%. Limited sequencing data was obtained from 2 events. Events lasted 60-150 minutes (singing: 20-120). Rooms ranged 320-3000m3. SARS-CoV-2 transmission likely occurred during all events; with a possible index case identified in 4 events. AirCoV2 showed 86% (54-100%) mean illness risk for 120 minutes of singing, smaller room (300m3), 1 air exchange/hour (ACH), and supershedder presence. ConclusionsDroplet transmission and indirect contact probably caused some cases, but unlikely explain the high attack rates. AirCoV2 indicated that airborne transmission due to singing is possible in case of supershedder presence. Airflow expelling respiratory droplets >1.5m possibly influenced transmission. It is possible that singing itself increased SARS-CoV-2 transmission risk through airborne transmission. SummaryThis outbreak investigation among five singing events with high SARS-CoV-2 attack rates (53-74%) suggested that airflow expelling respiratory droplets >1.5m possibly influenced transmission and it is possible that singing itself increased SARS-CoV-2 transmission risk through airborne transmission.