Trends in type 1 diabetes incidence between 2007 and 2023 and their association with SARS-CoV-2 infection in a population-based matched cohort study among individuals under 30 years old in Sweden.

AIMS/HYPOTHESIS The incidence of type 1 diabetes increased during the pandemic in various countries. SARS-CoV-2 infections may trigger the development of type 1 diabetes, but the evidence is inconclusive. This study aimed to assess trends in type 1 diabetes incidence between 2007 and 2023, and to quantify the association between SARS-CoV-2 infections and the risk for developing type 1 diabetes. METHODS The study included all individuals under 30 years old registered in Sweden. Deviations in type 1 diabetes incidence from pre-pandemic trends (2007-2019) were assessed for each pandemic year (2020-2023) using Poisson regression. The effect of SARS-CoV-2 infections was assessed using Cox proportional hazards models in a cohort of infected individuals with five control individuals from the infection date of the case, matched by birth year, sex and region. RESULTS Compared with the predicted linear trend, type 1 diabetes incidence increased by 12% during 2021 (incidence rate ratio [IRR] 1.12; 95% CI 1.06, 1.19) and 9% during 2022 (IRR 1.09; 95% CI 1.02, 1.16), but reverted to pre-pandemic trends in 2023. Overall, the adjusted HR for developing type 1 diabetes after SARS-CoV-2 infection was 0.96 (95% CI 0.79, 1.16). Children between 5 and 10 years old were more likely to develop type 1 diabetes within the first 28 days after infection (HR 2.68; 95% CI 1.22, 5.89), although their hazard over the whole follow-up period was not increased. CONCLUSIONS/INTERPRETATION Sweden, with its non-restrictive pandemic response, saw a transient increase in type 1 diabetes incidence that was only partially associated with SARS-CoV-2 infections. Other explanations should be investigated, including environmental and lifestyle factors.