COVID-19 cluster outbreaks and infection control measures in nursing homes in Okinawa: protocol for a retrospective...

Following the reclassification of COVID-19 as a category V infectious disease in Japan in May 2023, systematic public health monitoring of case numbers and cluster outbreaks was discontinued. Consequently, infection control measures, responses to cluster outbreaks and vaccination coverage in long-term care facilities were largely left to the discretion of individual facilities, and the understanding of the current situation in these settings remains insufficient. This observational study aims to retrospectively investigate COVID-19 cluster outbreaks and the resulting burdens experienced by facilities after reclassification, focusing on special nursing homes (Tokubetsu Yogo Rojin Home) for older adults, a population at particularly high risk for COVID-19-associated hospitalisation and mortality. By comprehensively evaluating facility and resident characteristics, vaccination coverage and infection prevention and control measures, this study seeks to generate evidence to inform future infection control strategies in long-term care facilities.

This retrospective observational study targets special nursing homes in Okinawa Prefecture and residents living in these facilities. Data for a target sample of nine nursing homes with approximately 900 residents will be collected from existing facility and medical records using the Research Electronic Data Capture system. The study was initiated in June 2025 and completion is anticipated by March 2027. The primary outcomes are the occurrence of COVID-19 cluster outbreaks in nursing homes between May 2023 and August 2025 and the facility-level burden experienced during outbreaks, including operational and economic impacts. A cluster outbreak is defined as the occurrence of ≥2 cases of the same infectious disease within a 10-day period. Key variables of interest include baseline characteristics of facilities and residents, vaccination coverage among staff and residents and infection prevention and control measures. Additionally, the study explores potential factors associated with residents' clinical outcomes and facility burden using logistic and linear regression models, respectively, with facility-level random intercepts if deemed appropriate. Prespecified supplementary analyses include one-way analysis of variance and χ² tests.

The study protocol was approved by the Institutional Review Board of the University of the Ryukyus in June 2025 (approval number: 25-2486-00-00-00). The findings will be disseminated at academic conferences and in peer-reviewed medical journals.