Summary

The potential influence of seasonal variations on vasculitis is unclear. Emerging evidence has suggested that seasonal factors may play a role in the onset of vasculitis. We extracted data from the electronic medical records at Clalit Health Services (CHS), Israel's largest health maintenance organization. We identified patients older than 18 years of age with new onset of giant cell arteritis (GCA), ANCA-associated vasculitis, immunoglobulin A (IgA) vasculitis, and Behçet's disease from 2007 to 2021. We constructed a time series of new vasculitis cases per month and explored the potential impact of seasonality on the disease onset. Our cohort included 4847 patients, including 2445 with GCA, 749 with ANCA-associated vasculitis (AAV), 547 with IgA vasculitis, and 1106 with Behçet's disease. We observed a decreased risk of GCA in September (relative risk [RR] 0.84, [95% confidence interval] 95%CI 0.72–0.98) and a significant reduction in AAV incidence in August (RR 0.68, 95%CI 0.48–0.96). For IgA vasculitis, an elevated risk was noted in February (RR 1.58, 95%CI 1.02–2.45), while Behçet's disease showed an increased risk in January (RR 1.25, 95%CI 1.02–1.55). No association was found between any specific season and the onset of vasculitis for any of the studied conditions. Our study results indicate that the onset of vasculitis conditions may be influenced by environmental factors associated with seasonality.