Incidence of Myocarditis Before and After SARS-CoV-2 Vaccination and Infection in Canterbury, 2016–2022

Abstract

To compare the incidence of myocarditis in Canterbury before the SARS-CoV-2 pandemic, during a period of high vaccination uptake, and during a period of widespread community transmission. Adult patients admitted to Christchurch Hospital between 2016 and 2022 who received a diagnostic code of acute myocarditis (ICD10 codes I40, I41, and I51.4) were included. Demographic data, peak troponin concentration (hs-cTn), mortality, recent SARS-CoV-2 vaccination, or positive SARS-CoV-2 test were recorded. Incidence of myocarditis during background (2016–2020), vaccination (2021), and community transmission (2022) phases were compared using Pearson’s Chi-squared test. Background incidence of myocarditis admission was seven per 100,000 population/year (CI 6–8.2); mean age was 47 [SD 16] years; 36% were women; and there was one death <28 days of admission. Vaccination phase incidence was 8.5 per 100,000 (CI 6.2–11.6); mean age was 49 [SD 19] years; 46% were women; eight (20%) admissions were within 28 days of vaccination, and one person died. Community transmission phase incidence was 13.1 per 100,000 (CI 10.2–16.9); mean age was 50 [SD 21] years; 48% were women; 28 (44%) cases were associated with active SARS-CoV-2 infection; and four patients died—three from COVID-19. There was no variance in annual incidence of myocarditis during vaccine rollout compared with 2016–2020 (p=0.5). The annual incidence of myocarditis during the period of SARS-CoV-2 infection was higher than previous years (p=0.001). There was an increased incidence of myocarditis diagnosis when there were high rates of SARS-CoV-2 community transmission, almost half of which were associated with active SARS-CoV-2 infection. There was no increase in myocarditis incidence associated with vaccination alone.