The main goal of this study is to characterize presentation, clinical course, and long-term outcomes of myocarditis temporally associated with administration of mRNA-1273 (SPIKEVAX) COVID-19 vaccine.

This is an observational cohort study that combines data collected directly from healthcare providers (HCP) with existing retrospective real-world data as captured in clinical electronic health record (EHR) and administrative claims data. Vaccine exposure and case identification information will be obtained retrospectively from existing real-world data to identify cases of post-vaccine myocarditis (PVM) and ultimately vaccine-associated myocarditis (VAM) for potential study inclusion. Eligible participants will be identified and followed for up to 5 years until the end of the study period or loss to follow-up or death.

Inclusion Criteria:

• Participants with a diagnosis of myocarditis between December 18, 2020, and October 31, 2026, will be identified or who have a relevant combination of laboratory and clinical findings meeting the CDC case definition for probable or confirmed myocarditis will be included in the study.
• Participants will be required to have at least 30 days of medical history to assess SPIKEVAX exposure.

The source population for this study will include participants with a diagnosis of myocarditis or a relevant combination of laboratory and clinical findings meeting the Centers for Disease Control and Prevention (CDC) case definition for probable or confirmed myocarditis occurring during the study window for case ascertainment.