Changing Epidemiology of Myocarditis in Australia: A Population-Based Cohort Study.

: Myocarditis is a serious disease that has drawn increasing attention due to its association with COVID-19 and vaccination. This study investigates the epidemiology of myocarditis beyond the COVID-19 pandemic, including its incidence and outcomes over time. : We analyzed the population-wide retrospective data from the Admitted-Patient-Data-Collection database of patients admitted to hospitals in New South Wales (NSW), Australia, with a diagnosis of myocarditis from 2001 to 2022. The incidence of myocarditis, changing classification of myocarditis over time, and complications of myocarditis over time were all calculated. : There were 4071 patients diagnosed with their first episode of myocarditis, with a median age of 42 years old, and 66% were male. The incidence of myocarditis in NSW has tripled over 20-years to 8.3 per-100,000-persons by 2022. Reactive myocarditis (i.e., myocarditis within 30-days of a respiratory or digestive illness) accounted for 38% of first presentations of myocarditis. Post COVID-19 myocarditis, a subset of reactive myocarditis, accounted for 42% of myocarditis admissions since the onset of the COVID-19 pandemic in Australia. Eight percent of patients had a background history of malignancy, and 6% had a history of autoimmune disease. In-hospital mortality was 4.5% during the entire study period but has been falling by 11% per year. During follow up, most readmissions for myocarditis occurred within 6-months; with 5.1% recurrence at 6-months compared to only 6.7% at 5-years. : Myocarditis is an important condition with increasing incidence in Australia and with markedly changing characteristics in the pandemic and post pandemic era.