Determinants of COVID-19 outbreak size in elderly residential facilities in Okinawa Prefecture, Japan, April to June 2022

Objectives: COVID-19 outbreaks in residential facilities for the elderly can have severe consequences; however, effective preventive strategies remain under-evaluated. This study aimed to identify actionable, facility-level factors associated with outbreak size in such facilities in Okinawa, Japan. Methods: We conducted a questionnaire-based cross-sectional study of 78 residential facilities for the elderly that experienced confirmed COVID-19 outbreaks between April and June 2022. Facility-level data on infection-control practices, outbreak characteristics, and staff testing approaches were analyzed using negative binomial regression models to quantify factors associated with outbreak size. Results: Outbreaks detected via contact-based testing of staff were significantly smaller than those detected through routine staff reverse transcription polymerase chain reaction screening (adjusted relative risk [aRR]: 0.11; 95% confidence interval [CI]: 0.03-0.37). Resident mask-wearing was associated with smaller outbreak sizes (aRR: 0.40; 95% CI: 0.16-0.99). Routine screening identified only 16.7% of staff index cases despite being widely implemented, suggesting limitations in effectiveness. Conclusions: Risk-based, exposure-driven testing appears markedly more effective than fixed-interval screening for limiting outbreak size in residential facilities for the elderly. Implementation should consider both operational feasibility and support systems for frontline staff.