Increase in people's behavioural risks for contracting COVID-19 during the 2021 New Year holiday season: longitudinal survey of the general population in Japan.

OBJECTIVES: There has been no study in Japan on the predictors of risk for acquiring SARS-CoV-2 infection based on people's behaviour during the COVID-19 pandemic. The aim of this study was to document changes in risk behaviour during the New Year's holiday season in 2021 and to identify factors associated with high-risk behaviour for infection using a quantitative assessment tool. DESIGN: A longitudinal survey. SETTING: Multiphasic health check-ups for the general population in Iwate Prefecture. PARTICIPANTS: Serial cross-sectional data were obtained using rapid online surveys of residents in Iwate Prefecture from 4 to 7 December 2020 (baseline survey) and from 5 to 7 February 2021 (follow-up survey). The data in those two surveys were available for a total of 9741 participants. MAIN OUTCOME MEASURES: We estimated each individual's risk of acquiring SARS-CoV-2 infection based on the microCOVID calculator. We defined four trajectories of individual risk behaviours based on the probabilities of remaining at low risk, increasing to high risk, improving to low risk and persistence of high risk. RESULTS: Among people in the low-risk group in the first survey, 3.6% increased to high risk, while high risk persisted in 80.0% of people who were in the high-risk group at baseline. While healthcare workers were significantly more likely to be represented in both the increasing risk and persistently high-risk group, workers in the education setting were also associated with persistence of high risk (OR 2.58, 95% CI 1.52 to 4.39; p<0.001). CONCLUSIONS: In determining countermeasures against COVID-19 (as well as future outbreaks), health officials should take into account population changes in behaviour during large-scale public events.