Association of COVID-19 and Development of Type 1 Diabetes: A Danish Nationwide Register Study

OBJECTIVE To compare the incidence of type 1 diabetes (T1D) before and during the coronavirus disease 2019 (COVID-19) pandemic and determine whether severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is associated with T1D development. RESEARCH DESIGN AND METHODS All Danish residents aged <30 years free of diabetes from 2015 to 2021 were in-cluded. Individuals were followed from 1 January 2015 or birth until the development of T1D, the age of 30, the end of the study (31 December 2021), emigration, development of type 2 diabetes, onset of any cancer, initiation of immunomodulat-ing therapy, or development of any autoimmune disease. We compared the incidence rate ratio (IRR) of T1D using Poisson regression models. We matched each person with a SARS-CoV-2 infection with three control individuals and used a cause-specific Cox regression model to estimate the hazard ratio (HR). RESULTS Among 2,381,348 individuals, 3,579 cases of T1D occurred. The adjusted IRRs for T1D in each quarter of 2020 and 2021 compared with 2015–2019 were as follows: January– March 2020, 1.03 (95% CI 0.86; 1.23); January–March 2021, 1.01 (0.84; 1.22), April–June 2020, 0.98 (0.80; 1.20); April–June 2021, 1.34 (1.12; 1.61); July–September 2020, 1.13 (0.94; 1.35); July–September 2021, 1.21 (1.01; 1.45); October–December 2020, 1.09 (0.91; 1.31); and October–December 2021, 1.18 (0.99; 1.41). We identified 338,670 individuals with a positive SARS-CoV-2 test result and matched them with 1,004,688 control individuals. A SARS-2-CoV infection was not significantly associated with the risk of T1D development (HR 0.90 [95% CI 0.60; 1.35]). CONCLUSIONS There was an increase in T1D incidence during April–June 2021 compared with April–June 2015–2019, but this could not be attributed to SARS-CoV-2 infection.