LAUSR

Objectives At the end of 2020, many countries commenced a vaccination programme against SARS-CoV-2. Public health authorities aim to prevent, and interrupt, outbreaks of infectious disease in social care settings. We aimed to investigate the association between the introduction of the vaccination programme and the frequency and duration of COVID-19 outbreaks in Northern Ireland (NI). Study Design We undertook an ecological study using routinely available national data. Methods We used Poisson regression to measure the relationship between the number of RT-PCR confirmed COVID-19 outbreaks in care homes and, as a measure of community COVID-19 prevalence, the Office for National Statistics COVID-19 Infection Survey estimated number of people testing positive for COVID-19 in NI. We estimated the change in this relationship and estimated the expected number of care home outbreaks in the absence of the vaccination programme. A Cox proportional hazards model estimated the hazard ratio of a confirmed COVID-19 care home outbreak closure. Results Care home outbreaks reduced by two-thirds compared to expected following the introduction of the vaccination programme, from a projected 1,625 COVID-19 outbreaks (95% prediction interval 1,553 – 1,694) between 7 December 2020 and 28 October 2021 to an observed 501. We estimated an adjusted hazard ratio of 2.53 of the outbreak closure assuming a 21-day lag for immunity. Conclusions These findings describe the association of the vaccination with a reduction in outbreak frequency and duration across NI care homes. This indicates probable reduced harm and disruption from COVID-19 in social care settings following vaccination. Future research using individual level data from care home residents will be needed to investigate effectiveness of the vaccines and the duration of their effects.