Genome epidemiological and seroepidemiological study of the COVID-19 cluster in the nursing home

Coronavirus disease 2019 (COVID-19) clusters are serious for older adults in nursing homes. Several studies have analyzed patient epidemiology and trends in blood antibody titers after COVID-19 cluster outbreaks, but few comparative analyses of pre- and post-cluster outbreaks have been reported. This study aimed to investigate epidemiological surveys and whole-genome sequences of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in clinical specimens following a COVID-19 cluster occurrence in a nursing home. Routes and modes of infection were analyzed in the study participants using 'Pomegraph', a newly developed integrated graphical tool, in combination with epidemiological and genome information. Our findings showed that although the viruses were the same XBB subvariants of SARS-CoV-2 Omicron XBB.1.5 and XBB.1.16, they were imported from three different locations, based on the genome sequences. We evaluated the amount of antibodies and neutralizing antibody titers in the participants' sera pre- and post- the cluster outbreak. Before the cluster outbreak, no significant differences were observed between infected and uninfected participants in the period after vaccination or in the amounts of antibodies and neutralizing antibody titers, which is difficult to determine in general clusters. Our results facilitated the analysis of cluster factors such as pathogens, susceptible persons, and transmission.