Nationwide epidemiology of invasive meningococcal disease in Japan, 2017–2025: implications of post-COVID-19 mobility

BACKGROUND: Invasive meningococcal disease (IMD) is a severe but rare infection in Japan. Changes in population mobility before, during, and after the Coronavirus disease 2019 (COVID-19) pandemic may have influenced IMD trends; however, recent epidemiological patterns in Japan remain incompletely described. METHODS: National surveillance data from January 2017 to October 2025 were analyzed. Temporal, age-specific, and geographic patterns were evaluated across three phases: Phase 1 (2017-2019, pre-COVID-19), Phase 2 (2020-2022, pandemic period), and Phase 3 (2023-October 2025, post-pandemic period with relaxed preventive measures). The relationship between annual IMD case counts and inbound international travel was explored using Spearman's rank correlation. RESULTS: A total of 288 IMD cases were identified. Annual case numbers decreased markedly from 48 in 2019 to 1 in 2021 and subsequently increased to 66 in 2024 and 68 by October 2025. Case counts were significantly lower in Phase 2 than in Phase 1 (p = 0.0003) and significantly higher in Phase 3 than in Phase 2 (p < 0.0001). Older adults consistently accounted for the largest number of cases, and adolescents showed a clear rebound in Phase 3. Geographic patterns demonstrated substantial prefectural heterogeneity; Tokyo showed the largest proportional increase, whereas Osaka and the Kansai region did not show a corresponding increase. Importantly, no distinct short-term increase in IMD incidence was observed during the Osaka Expo 2025 period. IMD case counts and inbound international travel exhibited parallel temporal patterns over the study period (Spearman's ρ = 1.000). CONCLUSIONS: IMD incidence in Japan decreased during the COVID-19 pandemic and increased again following the relaxation of public health measures. The post-pandemic rebound occurred alongside recovery of social contact and population mobility, while most cases were domestically acquired, and no event-specific surge was detected during the Osaka Expo 2025 period. These findings suggest that multiple contextual factors, including societal reopening and travel-related dynamics, may have contributed to the observed trends; however, causal relationships cannot be determined from this ecological analysis.