Assessing countermeasures during a hepatitis A virus outbreak among men who have sex with men

BACKGROUND: A hepatitis A epidemic occurred among men who have sex with men (MSM) in Japan in 2017–2018. In this study, we employ a parsimonious mathematical model to epidemiologically investigate the dynamics of infection, aiming to evaluate the effectiveness of campaign-based interventions among MSM to raise awareness of the situation. METHODS: A mathematical model describing a mixture of human-to-human transmission and environmental transmission was fitted to surveillance data. Taking seasonally varying environmental transmission into account, we estimated the reproduction number of hepatitis A virus during the course of epidemic, and, especially, the abrupt decline in this reproduction number following campaign-based interventions. RESULTS: The reproduction number prior to the countermeasures ranged from 2.6 to 3.1 and then began to decrease following campaign-based interventions. After the first countermeasure, the reproduction number decreased, but the epidemic remained supercritical (i.e., R(t) > 1). The value of R(t) dropped well below one following the second countermeasure, which used web articles to widely disseminate information about the epidemic risk. CONCLUSIONS: Although the effective reproduction number, R(t), changes because of both intrinsic and extrinsic factors, the timing of the examined countermeasures against hepatitis A in the MSM population was consistent with the abrupt declines observed in R(t). Even without vaccination, the epidemic was brought under control, and risky behaviors may have been changed by the increase in situation awareness reached through web articles. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12976-021-00150-1.