Clusters of coronavirus disease 2019 in medical institutions and elderly care facilities in Okayama Prefecture

　Coronavirus disease 2019 has spread worldwide and has yet to be contained. The Japanese government has taken measures against the occurrence of clusters. However, there has little evaluation of the occurrence of the clusters and their changes. Therefore, we investigated the occurrence of the clusters in medical institutions and elderly care facilities in Okayama Prefecture.
　We compared the characteristics of the clusters that occurred in each of the wave between October 21, 2020, and September 30, 2021, by using the data published that interval. As a case study, we also evaluated the characteristics of positive patients at a medical institution over where a cluster occurred in case the fourth wave.
　The overall number of cluster outbreaks decreased with the spread of vaccination. In the fourth wave, there was a period in which the vaccination of staff members at medical institutions became widespread, while patients or facility users were unvaccinated, resulting in a longer convergence period and an increase in the number of positive cases among patients relative to the number of staff members.
　The impact of the spread of vaccination and the duration of immunity acquired after vaccination on the occurrence of clusters should be closely monitored in the future.

原著 (Original)