Impact of the 2018 Western Japan Heavy Rain Disaster on Anti-influenza Virus Drug Prescribing: A Longitudinal Analysis Using the National Health Insurance Claims Database.

No study has evaluated the relationship between heavy rain disasters and influenza by comparing victims and non-victims, and we investigated the association between the 2018 western Japan heavy rain disaster and influenza. All patients registered in the National Health Insurance Claims Database and treated in the Hiroshima, Okayama, and Ehime prefectures were included in this retrospective cohort study conducted 1-year post-disaster. A multivariate mixed-effects logistic regression analysis was used to assess the association between the disaster and anti-influenza drug prescribing. A difference-in-differences analysis was conducted to assess anti-influenza drug use for the 4-month period immediately before and every 4 months for a year post-disaster. This study included 6 176 300 individuals (victims: 36 076 [0.60%]); 2573 (7.1%) and 458 157 (7.4%) in the victim and non-victim groups, respectively, used anti-influenza drugs in the year following the flood. The victims were significantly more likely than non-victims to use anti-influenza drug (risk ratio 1.18; 95% confidence interval [CI] 1.12-1.42). The victims used significantly more anti-influenza drugs in the 4 months immediately post-disaster compared with just before the disaster (odds ratio 3.62; 95% CI 1.77-7.41). Anti-influenza drug use was higher among victims of the 2018 Western Japan heavy rain disaster than among non-victims.