Trends in the incidence of childhood type 1 diabetes after the COVID-19 lockdown in Finland: An observational study

BACKGROUND: While COVID-19 lockdowns have been systematically associated with increased incidence rate of type 1 diabetes, less is known about the disease rates after lockdowns. We compared the incidence of type 1 diabetes after the COVID-19 pandemic-related closure of society (i) to the time when lockdown was implemented and to (ii) the time before the pandemic in Finland. METHODS AND FINDINGS: The incidence of type 1 diabetes after the lockdown (09/2021-12/2022) was compared to the incidence during the lockdown (03/2020-08/2021) and before the lockdown (three 18-month terms 2014-2019) in Finnish children under the age of 15 years. Previous exposure to SARS-CoV-2 was identified by combining information on COVID-19 vaccination with data on SARS-CoV-2 antibodies analyzed in blood samples. The temporal cohort included 3,467 pediatric type 1 diabetes cases, distributed across post-lockdown (n = 586), lockdown (n = 785), and pre-pandemic (n = 2,096) periods. The incidence of type 1 diabetes after the lockdown was significantly lower than the incidence during the lockdown [incidence rate ratio (IRR = 0.85; 95% Cl [0.77,0.95]; p = 0.004)]. The rate of type 1 diabetes after the lockdown did not significantly differ from the time before lockdown (IRR = 1.00; 95% Cl [0.91,1.09]; p = 0.932). The frequency of children testing positive for infection-associated SARS-CoV-2 antibodies increased from 0.9% during the lockdown to more than 40% after the lockdown. Study limitations include a relatively short follow-up period after the lockdown and the lack of explicit modeling of the underlying long-term secular trend in type 1 diabetes incidence. CONCLUSIONS: The lockdown represents a peak in childhood type 1 diabetes incidence in Finland compared to both the pre-pandemic and post-lockdown periods. However, the reversion toward historical levels after the lockdown does not necessarily establish a return to the expected historical trajectory, nor can it fully distinguish a transient change from temporal compensation superimposed on the background secular trend. The contrasting patterns between type 1 diabetes incidence and SARS-CoV-2 antibody prevalence during and after the lockdown suggest an indirect rather than a direct role of the virus in triggering the observed changes in incidence.