In this study, the occurrence of myocarditis inpatient encounters (4,560) was 42.3% higher than that during 2019 (3,205). Peaks in myocarditis inpatient encounters during April–May 2020 and November 2020–January 2021 generally aligned with peaks in COVID-19 inpatient encounters.

The risk for myocarditis among patients with COVID-19 during March 2020–January 2021 was nearly 16 times as high as the risk among patients without COVID-19, with the association between COVID-19 and myocarditis being most pronounced among children and older adults. Further, in this cohort, approximately 40% of patients with myocarditis had a history of COVID-19.

These findings suggest an association between COVID-19 and myocarditis, although causality cannot be inferred from observational data, and are consistent with those from previous studies.

Before this report, the two largest known studies, in the United States and in Israel, also found that COVID-19 was strongly associated with myocarditis.

In this study, the association between COVID-19 and myocarditis was lowest for persons aged 25–39 years and higher among younger (<16 years) and older (≥50 years) age groups, a pattern that has not been previously described in age-stratified analyses and that warrants further investigation.

The risk difference for myocarditis between persons with and without COVID-19 was higher among males than among females, consistent with some earlier studies. The finding of a higher risk ratio among females than among males is novel. However, it likely reflects the low risk for myocarditis among female patients without COVID-19.

Although the exact mechanism of SARS-CoV-2 infection possibly leading to myocarditis is unknown, the pathophysiology is likely similar to that of other viruses

Among persons with COVID-19 and myocarditis, some myocarditis diagnoses might represent cases of multisystem inflammatory syndrome (MIS), particularly among children aged <16 years. 

Since the introduction of mRNA COVID-19 vaccines in the United States in December 2020, an elevated risk for myocarditis among mRNA COVID-19 vaccine recipients has been observed, particularly among males aged 12–29 years, with 39–47 expected cases of myocarditis, pericarditis, and myopericarditis per million second mRNA COVID-19 vaccine doses administered.

A recent study from Israel reported that mRNA COVID-19 vaccination was associated with an elevated risk for myocarditis, in the same study; a separate analysis showed that SARS-CoV-2 infection was a strong risk factor for myocarditis.

On June 23, 2021, the Advisory Committee on Immunization Practices concluded that the benefits of COVID-19 vaccination clearly outweighed the risks for myocarditis after vaccination. The present study supports this recommendation by providing evidence of an elevated risk for myocarditis among persons of all ages with diagnosed COVID-19.

What is already known about this topic?

Viral infections are a common cause of myocarditis. Some studies have indicated an association between COVID-19 and myocarditis.

What is added by this report?

During March 2020–January 2021, patients with COVID-19 had nearly 16 times the risk for myocarditis compared with patients who did not have COVID-19, and risk varied by sex and age.

What are the implications for public health practice?

These findings underscore the importance of implementing evidence-based COVID-19 prevention strategies, including vaccination, to reduce the public health impact of COVID-19 and its associated complications.

https://www.cdc.gov/mmwr/volumes/70/wr/mm7035e5.htm?fbclid=IwAR1mbzEO7ig60H7vtw4hINyW62-CbhtiX5iiBPGNHktYfS4SjjBlupsgU8s 

https://www.acc.org/latest-in-cardiology/journal-scans/2021/06/16/19/12/possible-association-between-covid-vaccine 

https://jamanetwork.com/journals/jama/fullarticle/2782900 

https://www.jacc.org/doi/10.1016/j.jcmg.2021.06.002