Infection prevention in residential care homes for the elderly: insights from the antibody landscape of COVID-19 after an explosive outbreak

Abstract The immunologic landscape of COVID-19 in residential care homes for the elderly (RCHEs) after a territory-wide Omicron surge in early 2022 is informative for current infection control practice. A cross-sectional seroepidemiology survey was conducted among 676 RCHE residents in Hong Kong in 2022. The surrogate virus neutralization test showed that the median percentage inhibition against BA.2 was 46.7 overall, with the mean increasing from 8.63 in February to 54.1 in April before levelling off till November. Profiles of hybrid immunity presented higher neutralizing antibody (nAb) levels than those with CoronaVac (C) or infection (I). A generalized additive regression analysis, accounting for the time elapsed between the last immunologic exposure and blood collection, showed that recent infection profiles (CCCI, CCIC) were associated with higher nAb levels than ICCC. Pearson correlation analysis uncovered that the cross-reactivity between BA.2 and BA.4/5 diverged by serostatus. By pairing cross-sectional sera with vaccination and infection histories, this study elucidates how the immunity landscape in outbreak-prone RCHEs was shaped by immunologic exposures. The moderate post-outbreak nAb level hints at a suboptimal immunity level in RCHEs in the post-Omicron period. The findings strengthen the evidence base for retaining vaccination as an infection control strategy, and provide immunologic support for its time-dependent effectiveness.