Background
The COVID-19 pandemic significantly impacted care homes, highlighting their vulnerability to infection. I described the burden of infection and investigated facility-level risk factors for SARS-CoV-2 infections and outbreaks within care homes.

Methods
I helped to rapidly establish the VIVALDI cohort study in ~330 care homes for older people in England (ISRCTN14447421), which hosted my analyses. I reviewed the literature to investigate risk factors for SARS-CoV-2 in care homes. Using data from asymptomatic SARS-CoV-2 testing and anti-nucleocapsid (from infection) and anti-spike (from vaccination) antibodies in care home staff and residents, I estimated prevalence and spread of SARS-CoV-2 infection across homes and modelled longevity of antibody responses following infection and vaccination. Finally, I designed a built environment survey and evaluated environmental risk factors for ingress and transmission of SARS-CoV-2.

Results
Within VIVALDI, over one-quarter of staff and one-third of residents were infected over 15 months from the pandemic start, increasing to two-thirds after two years. I showed that nucleocapsid-antibodies were negative in half of participants eight months post-infection, suggesting waning immunity, however spike-antibody waning rates following vaccination were comparable between staff and residents. I demonstrated rapid spread of the emergent B.1.1.7 variant in care homes, suggesting introduction of infection from the community. Community incidence of SARS-CoV-2 was also the main risk factor for infection ingress (measured by outbreak incidence) but not transmission (measured by infection incidence, outbreak size, and duration), which was associated with environmental factors like bedroom and storey number, building type, indoor temperature, air quality, and ventilation.

Conclusion
Care homes experienced high SARS-CoV-2 rates despite stringent control measures, with comparable antibody responses between staff and residents that wane following infection. Although preventing infection entry is challenging, environmental modifications may limit spread. Building on lessons from VIVALDI, controlling infection in care homes should be a research priority.