The epidemiology of myocarditis is largely unknown and based mainly on small single-center studies. The study aim to evaluate the current incidence, clinical characteristics and outcomes of patients hospitalized due to myocarditis in a general population.

In this retrospective-prospective study, data from the National Health Fund (NHF), which is the only public healthcare insurer in Poland is used. The NHF reimburses drugs and healthcare services provided by healthcare providers (both public and private) with public funds collected from health insurance premiums. In Poland, public health insurance is obligatory for almost all Poles - in December 2019, 88.4% of approx. 38 million Poles had public health insurance and were entitled to obtain healthcare services and drugs reimbursed by the NHF.

Based on NHF claims data, the healthcare services reported over the years 2009-2021 with a diagnosis of myocarditis - hospitalizations reported with codes I40, I40.0, I40.1, I40.8, I40.9, I41, I41.0, I41.1, I41.2, I41.8, I51.4, B33.2 are derived according to the International Classification of Diseases and Related Health Problems, 10th Revision (ICD-10). The dataset is narrowed to the first myocarditis hospitalization, i.e. patients for whom no information about the diagnosis of myocarditis is reported in the 400 days preceding the hospitalization. For such a group of patients, and to establish the baseline characteristics of the patients, data is analyzed 400 days back from the initial diagnosis of myocarditis. Also, in-hospital and long-term outcomes are analyzed, including all-cause mortality as well as the occurrence of selected diseases (defined as receiving a service where selected ICD-10 was reported), and selected procedures are obtained, defined with codes according to International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM). For the follow-up, at least a six-month period is required.

To show differences related to the age of patients hospitalized due to myocarditis, baseline characteristics and long-term outcomes are assessed with regard to age groups.

Data from the Central Statistical Office of Poland is also used to refer the obtained results to the population of Poland and to obtain life tables for survival analysis.

No specific exclusion criteria were applied. No ethics approval was required for this study, as it involved the analysis of administrative data.

Inclusion Criteria:

• first myocarditis hospitalization

Exclusion Criteria:

• no specific exclusion criteria were applied

All patients with first myocarditis hospitalization in Poland in observed time period.

data can be provided upon request