Effects of 2018 Japan floods on healthcare costs and service utilization in Japan: a retrospective cohort study

BACKGROUND: Floods and torrential rains are natural disasters caused by climate change. Unfortunately, such events are more frequent and are increasingly severe in recent times. The 2018 Japan Floods in western Japan were one of the largest such disasters. This study aimed to evaluate the effect of the 2018 Japan Floods on healthcare costs and service utilization. METHODS: This retrospective cohort study included all patients whose receipts accrued between July 2017 and June 2019 in Hiroshima, Okayama, and Ehime prefectures using the National Database of Health Insurance Claims. We used Generalized Estimating Equations (GEEs) to investigate yearly healthcare costs during the pre-and post-disaster periods, quarterly high-cost patients (top 10%), and service utilization (outpatient care, inpatient care, and dispensing pharmacy) during the post-disaster period. After the GEEs, we estimated the average marginal effects as the attributable disaster effect. RESULTS: The total number of participants was 5,534,276. Victims accounted for 0.65% of the total number of participants (n = 36,032). Although there was no significant difference in pre-disaster healthcare costs (p = 0.63), post-disaster costs were $3,382 (95% CI: 3,254–3,510) for victims and $3,027 (95% CI: 3,015–3,038) for non-victims (p < 0.001). The highest risk difference among high-cost patients was 0.8% (95% CI: 0.6–1.1) in the fourth quarter. In contrast, the highest risk difference of service utilization was in the first quarter (outpatient care: 7.0% (95% CI: 6.7–7.4), inpatient care: 1.3% (95% CI: 1.1–1.5), and dispensing pharmacy: 5.9% (95% CI: 5.5–6.4)). CONCLUSION: Victims of the 2018 Japan Floods had higher medical costs and used more healthcare services than non-victims. In addition, the risk of higher medical costs was highest at the end of the observation period. It is necessary to estimate the increase in healthcare costs according to the disaster scale and plan for appropriate post-disaster healthcare service delivery. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12889-023-15205-w.