Seasonal Variation in Antineutrophil Cytoplasmic Antibody (ANCA)‐Associated Vasculitis Onset Before and During the Coronavirus Disease 2019 (COVID‐19) Pandemic: Findings From the Multicenter J‐CANVAS Registry

AIM: To investigate seasonal patterns in new-onset antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) during the coronavirus disease 2019 (COVID-19) pandemic in Japan. METHODS: We conducted a retrospective cohort study using data from the nationwide multicenter registry of patients with AAV. Patients newly diagnosed with AAV between 2017 and 2022 were included, comprising microscopic polyangiitis (MPA), granulomatosis with polyangiitis (GPA), and eosinophilic granulomatosis with polyangiitis (EGPA). We compared the seasonal distribution of AAV onset between the pre-COVID-19 (2017-2019) and COVID-19 (2020-2022) periods. RESULTS: A total of 871 patients were included (499 before and 372 during the COVID-19 period). Poisson regression analysis revealed a seasonal variation in AAV onset in the pre-COVID-19 period (p < 0.001), with lower onset in autumn [incidence rate ratio (IRR) 0.64, p < 0.001] and higher onset in spring and summer (IRR 1.16, p = 0.048 and IRR 1.23, p = 0.005) compared to winter. Importantly, a significant interaction between season and period (p for interaction = 0.032) indicated that the seasonal pattern differed between periods, with a relative increase in spring (IRR 1.16, p = 0.010). In the subanalyses, seasonal variation was consistently observed in MPA, but not in GPA and EGPA. The interaction between season and period was not statistically significant in each disease group. CONCLUSION: AAV exhibited a clear seasonal pattern, which differed during the COVID-19 period. However, these differences were not consistently observed across individual disease subtypes, and the factors underlying them remain unclear.