Genome-Epidemiological and Seroepidemiological Study of the COVID-19 Cluster in a Nursing Home.

Coronavirus disease 2019 (COVID-19) clusters pose a serious risk 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 the periods both before and after 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 among the study participants were analyzed using "Pomegraph," a newly developed integrated graphical tool, in combination with epidemiological and genome information. Our findings showed that, although all the viruses belonged to the Omicron XBB.1.5 or XBB.1.16 subvariants of SARS-CoV-2, 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 before and after 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 levels of antibodies and neutralizing antibody titers, which are difficult to determine in investigations of general clusters. Our results facilitated the analysis of cluster factors, such as pathogens, susceptible individuals, and transmission.