Unraveling the Modes of Transmission of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) During a Nursing Home Outbreak: Looking Beyond the Church Superspreading Event.

Background: An outbreak of coronavirus disease 2019 (COVID-19) in a nursing home in the Netherlands, following an on-site church service held on 8 March 2020, triggered an investigation to unravel sources and chain(s) of transmission.

Methods: Epidemiological data were collected from registries and through a questionnaire among church attendees. Symptomatic residents and healthcare workers (HCWs) were tested for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) by reverse-transcription polymerase chain reaction and subjected to whole genome sequencing (WGS). Sequences from a selection of people from the same area were included as community reference.

Results: After the church service, 30 of 39 attendees (77%) developed symptoms; 14 (11 residents and 3 nonresidents) were tested and were positive for COVID-19. In the following 5 weeks, 62 of 300 residents (21%) and 30 of 640 HCWs (5%) tested positive for COVID-19; 21 of 62 residents (34%) died. The outbreak was controlled through a cascade of measures. WGS of samples from residents and HCWs identified a diversity of sequence types, grouped into 8 clusters. Seven resident church attendees all were infected with distinct viruses, 4 of which belonged to 2 larger clusters in the nursing home.

Conclusions: Although initial investigation suggested the church service as the source of the outbreak, detailed analysis showed a more complex picture, most consistent with widespread regional circulation of the virus in the weeks before the outbreak, and multiple introductions into the nursing home before the visitor ban. The findings underscore the importance of careful outbreak investigations to understand SARS-CoV-2 transmission to develop evidence-based mitigation measures.