COVID-19 outbreaks in residential care homes in Hong Kong and effectiveness of vaccine against severe outcomes.

Background: COVID-19 outbreaks in residential care homes for the elderly (RCHEs) and for persons with disability (RCHDs) have caused significant morbidity and mortality during 5th epidemic in Hong Kong. This article reviewed COVID-19 outbreaks situation and estimated the effectiveness of receiving at least two-dose of COVID-19 vaccine in preventing severe outcomes. Methods: To estimate attack rates and vaccination coverage, documentation on COVID-19 infection and their vaccination records of residential care homes (RCH) residents reported between December 31, 2021 and May 31, 2022 were reviewed, and infected cases were follow-up for 4 weeks for severe outcomes or death. Correlation between vaccination coverage against attack rate by types of homes was examined. Infected RCH residents with available information were included in the analysis of vaccine effectiveness against severe outcomes and death. Results: COVID-19 vaccination coverage was low in RCHDs (median 0.46, IQR: 0.24-0.76) and very low in RCHEs (median 0.08, IQR: 0.00-0.19). Higher attack rates were recorded among RCHE residents (median 0.84, IQR: 0.64-0.93) and higher case fatality rate (CFR: 28.1%) than in RCHDs (median 0.58, IQR: 0.31-0.84; CFR: 3.9%). The attack rate decreased when vaccination coverage increased for both RCHEs (ρ = -0.131, p < 0.001) and RCHDs (ρ = -0.333, p < 0.001). Comparing with infected residents who were unvaccinated/vaccinated with one-dose, receiving at least two-dose was estimated to be effective in reducing severe outcomes in 31% and 36% of infected RCHE and RCHD residents respectively; with greater reduction in mortality among RCHD than RCHE residents (54% and 38%, respectively). Vaccine effectiveness of two-dose of BNT162b2 against severe outcomes and death are higher than that of CoronaVac. Conclusions: Increasing COVID-19 vaccination could have significant impact on reducing the risk of COVID-19 outbreaks in RCHs. At least two-dose of COVID-19 vaccine is still effective in reducing severe outcomes and death among infected residents in RCHs during Omicron epidemic.