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Mounting data on mRNA vaccination against COVID-19 continued to suggest a small excess risk of myocarditis, especially in young men and adolescent boys.

Based on voluntary reports to the passive surveillance Vaccine Adverse Event Reporting System (VAERS), the number of reported cases of myocarditis after mRNA vaccination from December 2020 to August 2021 exceeded what could be expected for several groups, noted Matthew Oster, MD, MPH, of the CDC in Atlanta, and colleagues.

Myocarditis occurring within 7 days after the second vaccine dose was most common in:

• Adolescent males ages 16 to 17: 105.9 per million doses of the Pfizer vaccine
• Adolescent males ages 12 to 15: 70.7 per million doses of the Pfizer vaccine
• Young men ages 18 to 24: 52.4 and 56.3 per million doses of Pfizer and Moderna vaccines, respectively

Even though nearly all people with post-vaccination myocarditis were hospitalized and clinically monitored (95%), they typically experienced symptomatic recovery by discharge (87%) after receiving only nonsteroidal anti-inflammatory drugs (87%), the investigators reported .

They deemed myocarditis a "rare but serious adverse event" and urged the public to weigh the benefits of COVID-19 vaccination against the risks.

This study confirms previous reports from the CDC and others about the higher risk of myocarditis in boys and young men, the higher risk after the second vaccine dose, decreasing risk with age, and self-limited symptoms that resolve with supportive measures, commented Biykem Bozkurt, MD, PhD, a cardiologist at Baylor College of Medicine in Houston.

Of note, they found that the highest risk of myocarditis was in the age 16-17 group, not the 12-15 group, raising the possibility that the mechanisms are related to testosterone, dysregulated immune response, or genetic predisposition in certain patients, in addition to molecular mimicry and autoantibody production, Bozkurt said.

Oster and colleagues acknowledged two VAERS reports on mRNA-vaccinated people younger than 30 who died with potential myocarditis; however, these cases remain under investigation and were not included in the paper's case counts.

"Given the high verification rate of reports of myocarditis to VAERS after mRNA-based COVID-19 vaccination, underreporting is more likely. Therefore, the actual rates of myocarditis per million doses of vaccine are likely higher than estimated," they pointed out.

Cardiologist Harlan Krumholz, MD, SM, of Yale School of Medicine in New Haven, Connecticut, emphasized this limitation of passive surveillance.

"The take home message across studies, however, is quite consistent: myocarditis does seem associated with the mRNA vaccines, the rates are quite low, and the major complications even lower. Young men and adolescent males, especially after the second dose, have the highest risk, but it is still quite low," he wrote in an email.

CDC has started active follow-up surveillance to assess cardiac outcomes in young people 3 to 6 months after their VAERS report of myocarditis, Oster's group noted.

At the time of the study, VAERS had captured 1,991 reports of myocarditis -- 1,626 meeting the case definition of myocarditis -- after COVID-19 vaccination in over 192 million people older than 12.

Myocarditis patients had a median age of 21 years; boys and men comprised 82% of cases.

Time to symptom onset was approximately 2 days, and 90% of myocarditis events reportedly occurred within 7 days of the second dose.

Myocarditis rates in women and girls stayed lower than those in male peers under 50 years of age.

For persons younger than 30, Oster and colleagues reported some clinical information that was available from medical record reviews and clinician interviews:

• Symptoms included chest pain, pressure, or discomfort in 89% of cases
• Troponin levels were elevated in 98% of those tested
• Abnormal results were observed on 72% of ECGs
• Abnormal results were observed on 72% of cardiac MRI scans

"In contrast, typical viral cases of myocarditis can have a more variable clinical course. For example, up to 6% of typical viral myocarditis cases in adolescents require a heart transplant or result in mortality," they noted.

They cautioned that VAERS reports are of variable quality and tend to lack key clinical information, such as prior SARS-CoV-2 infection.

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