Epidemiology of myocarditis following COVID-19 or influenza and use of diagnostic assessments.

Background: Previous research has suggested a heightened risk of acute myocarditis after COVID-19 infection. However, it is not clear from existing work whether this risk is higher than would be expected after comparable viral respiratory infections. This information is important to guide risk assessments and clinical practice.

Methods: A retrospective cohort study of US administrative health claims was conducted to compare the rates of myocarditis after COVID-19 with that after influenza infection and describe the clinical use of diagnostic assessments.Patients with either incident COVID-19 diagnosis (between 1 January 2020 and 31 December 2021) or incident influenza diagnosis (between 1 January 2016 and 31 December 2018), with at least 12 months of continuous enrolment prior to index date and without a previous diagnosis of myocarditis were included.The primary outcome was clinically diagnosed acute myocarditis recorded after COVID-19 or influenza infection. Results are reported as covariate-adjusted subdistribution HRs from competing risk regression with COVID-19 considered as the exposure of interest and influenza as the reference group. Death was considered a competing risk.

Results: 1 120 760 adult COVID-19 patients and 439 278 adult influenza patients were identified, of which 669 (0.06%) adult COVID-19 patients and 91 (0.02%) adult influenza patients received a diagnosis of myocarditis. The myocarditis rate per 1000 person-years was 0.73 (95% CI 0.67 to 0.78) for adult COVID-19 patients and 0.24 (95% CI 0.19 to 0.28) for adult influenza populations. In models comprehensively adjusted for demographic and clinical risk factors, COVID-19 diagnosis (compared with influenza diagnosis), cardiac comorbidities, being male and under the age of 30 were independently associated with an increased risk of myocarditis in the year after diagnosis.

Conclusions: These findings support a distinct link between COVID-19 and myocarditis, which appears greater than after a similar viral respiratory infection. As such, a greater degree of clinical suspicion and investigation according to existing diagnostic pathways is recommended.