Infection and transmission of SARS-CoV-2 in London care homes reporting no cases or outbreaks of COVID-19: prospective observational cohort study, England 2020

Jeffery-Smith, Anna; Dun-Campbell, Kate; Janarthanan, Roshni; Fok, Jonathan; Crawley-Boevey, Emma; Vusirikala, Amoolya; De Olano, Elena Fernandez Ruiz; Perez, Marina Sanchez; Tang, Suzanne; Rowland, Thomas AJ; Wynne-Evans, Edward; Bell, Anita; Patel, Bharat; Amin-Chowdhury, Zahin; Aiano, Felicity; Paranthaman, Karthik; Ma, Thomas; Saavedra-Campos, Maria; Ellis, Joanna; Lackenby, Angie; Whitaker, Heather; Myers, Richard; Höschler, Katja; Brown, Kevin; Ramsay, Mary E; Shetty, Nandini; Yimmy Chow, J.; Ladhani, Shamez; Zambon, Maria

Background Care homes have been disproportionately affected by the COVID-19 pandemic. We investigated the potential role of asymptomatic infection and silent transmission in London care homes that reported no cases of COVID-19 during the first wave of the pandemic. Methods Five care homes with no cases and two care homes reporting a single case of COVID-19 (non-outbreak homes) were investigated with nasal swabbing for SARS-CoV-2 RT-PCR and serology for SARS-CoV-2 antibodies five weeks later. Whole genome sequencing (WGS) was performed on RT-PCR positive samples. Serology results were compared with those of six care homes with recognised outbreaks. Findings Across seven non-outbreak homes, 718 (387 staff, 331 residents) individuals had a nasal swab and 650 (385 staff, 265 residents) had follow-up serology. Sixteen individuals (13 residents, 3 staff) in five care homes with no reported cases were RT-PCR positive (care home positivity rates, 0 to 7.6%) compared to 13 individuals (3.0 and 11.1% positivity) in two homes reporting a single case. Seropositivity across these seven homes varied between 10.7-56.5%, with four exceeding community seroprevalence in London (14.8%). Seropositivity rates for staff and residents correlated significantly (rs 0.84, [95% CI 0.51-0.95] p <0.001) across the 13 homes. WGS identified multiple introductions into some homes and silent transmission of a single lineage between staff and residents in one home. Interpretation We found high rates of asymptomatic infection and transmission even in care homes with no COVID-19 cases. The higher seropositivity rates compared to RT-PCR positivity highlights the true extent of the silent outbreak. Funding PHE