Background: During the coronavirus disease 2019 (COVID-19) pandemic, the seasonality of Kawasaki disease (KD) in Japan shifted from winter dominance to summer dominance. However, it remains unclear whether this pattern reverted after infection mitigation measures were lifted in 2023. Methods: We conducted a population-based retrospective survey among all hospitalized patients with KD in Shiga Prefecture, Japan, from 2015 to 2024. Monthly variations in the KD incidence from 2021 to 2024 were analyzed using the 2015–2019 period as reference. We also examined the seasonality of 10 common pediatric infectious diseases and seasonal influenza during the study period. Results: A total of 2077 patients with KD were identified. The KD incidence remained low during the implementation of public health measures against COVID-19 from 2020 to 2022. Subsequently, the incidence reached the maximum in 2023 (128.2 per 100,000 person-years), coinciding with the lifting of infection mitigation measures; the incidence remained high at 118.9 in 2024. The traditional winter peak in KD incidence disappeared after 2020. Instead, peak incidence was observed in the summer from 2021 to 2022 and in the spring from 2023 to 2024. The number of patients with respiratory syncytial virus infection, which had been highest during the fall and winter from 2015 to 2019, reached the highest levels during the spring and summer from 2021 onward. Conclusions: KD seasonality in Shiga Prefecture did not revert to the traditional winter-dominant pattern after COVID-19 infection mitigation measures were lifted. Continued surveillance is warranted to determine whether the winter-dominant pattern re-emerges.
Hoshino et al. (Wed,) studied this question.