Epidemiology of Kawasaki Disease After the COVID-19 Pandemic in Japan 2023-2024

Importance: The COVID-19 pandemic may serve as a natural experiment, with pandemic-associated changes in the epidemiology of Kawasaki disease (KD) potentially informing understanding of KD pathogenesis. Objectives: To assess long-term KD incidence across 50 years and to examine pandemic-associated changes in KD epidemiology across age groups. Design, Setting, and Participants: This cohort study was a descriptive epidemiologic analysis of data from the Japanese Nationwide Survey of Kawasaki Disease conducted from 2023 to 2024, along with 50 years of historical data. Data were included from patients with KD in the current 2023-2024 survey (n = 29 841) and historical and current survey data from 1975 through 2024 (n = 467 456) for trend analysis. Main Outcomes and Measures: Annual KD incidence overall and stratified by 3 age groups: infants (<1 year), 1 to 4 years, and 5 to 9 years. Results: The current survey registered 15 032 and 14 809 patients with KD in 2023 and 2024, respectively (overall, 57.3% male; median IQR age, 2 1-4 years). Across the 50-year study period, despite the population under 5 years of age declining in Japan by 61.6%, KD incidence rates increased 16.1-fold. Across the COVID-19 pandemic period, patient numbers reached a nadir of 10 333 in 2022, then rebounded to 15 032 cases (45.5% increase) in 2023. Notably, decline and rebound patterns differed by age. Indexed incidence rates (using 2017 as a baseline of 100) demonstrated that patients 5 to 9 years of age had the largest decline (nadir of 54.2 in 2021, 45.8% below baseline) and the largest rebound (134.8 in 2024, 34.8% above baseline). In contrast, patients younger than 1 year showed the smallest decline (nadir of 73) and incidence rates did not return to baseline levels. When comparing the magnitude of rebound in KD incidence, a distinct age-dependent dose-response pattern was observed: 26.1% for infants, 38.9% for patients aged 1 year, 42.7% for patients aged 2 years, 85.8% for patients aged 3 years, 105.3% for patients aged 4 years, and 166.7% for patients aged 5 to 9 years. Conclusions and Relevance: In this nationwide cohort study of KD in Japan, KD incidence rebounded after the relaxation of COVID-19 pandemic-related restrictions, with age-dependent patterns. Nonpharmaceutical interventions (eg, mask-wearing) during the pandemic may explain the greater rebound pattern observed in older children; however, the minimal changes in infants suggest different exposure pathways to potential KD triggers. These findings support an age-stratified approach to KD pathogenesis research, distinguishing infants from older children.