Objective: In 2018, the Shinjuku City Department of Health detected excess cases of hepatitis A virus (HAV) infection. The objectives of this investigation were to characterize the outbreak, identify transmission routes among inpatient cases and make recommendations to control and prevent HAV infection among men who have sex with men. Methods: Information about cases of HAV infection was collected from the National Epidemiological Surveillance for Infectious Diseases system and inpatient interviews conducted by public health nurses in 2018. Results: There were 131 HAV cases in 2018. Of these, 98% (129/131) were male, of whom 81% (105/129) were men who have sex with men. Hospitalization was required for 40 cases (31%). The age groups with the highest proportion of cases were 30–39 and 40–49 years (each 34%; 44/131). Two cases (2%) had received the second dose of the HAV vaccine, but only 10 days before symptom onset; all others had received no doses. The sequence type subgroup 13, an RIVM-HAV-16–090-like strain, was seen in 51 cases (39%). Of the 40 hospitalized cases, 21 (53%) participated in an interview conducted using a semistructured questionnaire. Altogether, of 21 cases, 12 (57%) had coinfection with HIV, 13 (62%) had casual sexual contact within the preceding 2 months and 10 (48%) had used social networking services (SNS) to find a sexual partner. Discussion: In Shinjuku, this outbreak almost exclusively affected the population of men who have sex with men. The detected outbreak strain has previously been reported in outbreaks among men who have sex with men in Taiwan (China) and Europe. For HAV prevention, the most important measures are raising awareness of the risk of HAV as a sexually transmitted infection via SNS and promoting immunization at the appropriate time.

The Tokyo 2020 Olympic and Paralympic Games (the Games) were held from 23 July to 5 September 2021 in Tokyo, Japan, after a 1-year delay due to the coronavirus disease (COVID-19) pandemic. The Tokyo Metropolitan Government was responsible for monitoring and responding to infectious disease outbreaks other than COVID-19 during the Games. A multi-source surveillance system was used from 1 July to 12 September 2021 for the early detection and rapid response to infectious diseases. This included routine notifiable disease surveillance, sentinel surveillance, syndromic surveillance, cluster surveillance, ambulance transfer surveillance and the Tokyo Infectious Alert system. Daily reports were disseminated summarizing the data collected from the multi-source surveillance system. No case of infectious disease under the Tokyo Metropolitan Government system required a response during the Games. The multi-source surveillance was useful for providing intelligence during the Games and, if required, could contribute to the early detection and rapid response to outbreaks during other mass gatherings. The system could be improved to overcome the challenges implied by the findings of this multi-source surveillance.