Increased incidence of idiopathic inflammatory myopathies after SARS-CoV-2 outbreak: a French interrupted time-series study using the French national health data system and myositis-specific autoantibody data

Objectives Idiopathic Inflammatory Myopathies or myositis constitute a group of rare and severe autoimmune diseases classified in four subgroups based on various pathophysiological mechanisms. Although the factors triggering these diseases remain poorly understood, some environmental factors, such as respiratory viral infections, are suspected. The abrupt 2020 SARS-CoV-2 outbreak represents a quasi-experimental setting to investigate this hypothesis. Methods An ecological study by interrupted time series was conducted during the 2018-2022 period, in two complementary databases. First, all hospitalizations for incident myositis in France were extracted from the French National Health Data System (SNDS). Then, results of Myositis-Specific Autoantibody (MSA) tests performed in 14 French academic immunology laboratories were compiled (MSA database) in order to identify the different myositis subgroups. Breakpoints in incident cases of myositis and in first positive MSA tests were identified by minimization of a penalized cost function and tested by a Chow test. Pre- and post-mean number of incident cases and of first positive MSA tests were compared by a Student's t-test. Results In both series, we identified a breakpoint in 11/2020, respectively 8 and 3 months after the first and second SARS-CoV-2 peaks in France, followed by an increase of the mean number of new cases. The analysis of the MSA database concluded in an increase of the mean number of first positive MSA globally and in each subgroup with a maximal increase in dermatomyositis. Conclusion This study provides a new argument for the pathogenic role of viral infection in autoimmunity triggering, here applied to myositis.