Young Men, Adolescents at Raised Risk for COVID-19 mRNA Vaccine Myocarditis

Matthew E. Oster, MD, MPH

Risk of myocarditis following vaccination with either available messenger RNA (mRNA) vaccine for COVID-19 is significantly low, but altogether higher than expected in persons aged 24 years and younger, according to a new national study.

A team of investigators from the Centers for Disease Control and Prevention (CDC) reported findings from the Vaccine Adverse Event Reporting System (VAERS) showing that cases of the inflammatory heart event occurred 1626 times after an mRNA vaccine dose in US individuals from December 2020 to August 2021.

Contrasted against the 354,100,845 total doses of mRNA vaccine doses administered in that time frame, the prevalence of myocarditis is significantly low (.0000046%), three-fourths of the observed cases occurred in vaccinated persons younger than 30 years old, and one-third in those younger than 21 years old.

The research team, led by Matthew E. Oster, MD, MPH, of the CDC and the Emory University School of Medicine, additionally found differing rates of myocarditis risk in mRNA-vaccinated individuals dependent on gender and total doses received. They hope the findings can contribute to continued benefit-risk discussion for young adults and adolescents eligible for COVID-19 vaccination.

Oster and colleagues sought to describe the reports and confirmed cases of myocarditis available in the national passive surveillance VAERS database among individuals vaccinated with either Pfizer-BioNTech’s BNT162b2 or Moderna’s mRNA-1273, to provide estimated risks of myocarditis after vaccination based on age, sex and vaccine type.

The concept of vaccination serving as a trigger for myocarditis—a condition most commonly observed in infancy, adolescence or young adulthood—is mostly hypothesis, investigators wrote; only the smallpox vaccine has been causally linked to the cardiovascular event based on clinical reports. But the widespread and unprecedentedly expedited COVID-19 vaccination rollout implemented in December 2020 following the US Food and Drug Administration (FDA) Emergency Use Authorization (EUA) of BNT162b2—the first ever regulated mRNA vaccine in the US—warranted monitoring of special-interest adverse events via VAERS.

“As the reports of myocarditis after COVID-19 vaccination were reported to VAERS, the Clinical Immunization Safety Assessment Project, a collaboration between the CDC and medical research centers, which includes physicians treating infectious diseases and other specialists (eg, cardiologists), consulted on several of the cases,” investigators wrote. “In addition, reports from several countries raised concerns that mRNA-based COVID-19 vaccines may be associated with acute myocarditis.”

Investigator adjudicated and summarized reports of myocarditis to VAERS for all age groups through the observed time. Rates of myocarditis were calculated and stratified by age and sex of patient. They additionally calculated rates of myocarditis by age and sex for 2017-2019 through claims data. Clinical presentation, diagnostic test results, treatment and early outcomes of patients aged ≤30 years old were conducted.

Their assessment included 192,405,448 individuals who received 354,100,845 doses of either mRNA COVID-19 vaccine. Of the 1991 reports of myocarditis in COVID-19 mRNA vaccine recipients from the observed time, 1626 met the case definition of the condition.

Median patient age for myocarditis patients was 21 years old (IQR, 16 – 31) and median time to symptom onset was 2 days (IQR, 1 -3) post-vaccine dose. Among eligible reports that included patient sex, men comprised 82% of all observed myocarditis cases.

Investigators noted that the crude reporting rates for cases of myocarditis within 7 days of COVID-19 mRNA vaccination exceeded expectation across multiple strata of age and sex. Approximately 71 per 1 million doses of BNT162b2 resulted in myocarditis among adolescent males aged 12 – 15 years old; the rate was even greater in males aged 16 – 17 years old (105.9 per 1 million). The rate remained significant for men aged 18 – 24 years old for both vaccines (BNT162b2, 52.4 per 1 million; mRNA-1273,56.3 per 1 million).

Of the 826 cases observed among adults ≤30 years old, a majority (98%) were shown to include elevated troponin levels, as well as abnormal electrocardiogram results (72%) and abnormal cardiac magnetic resonance imaging (MRI) results (72%). Nonsteroidal anti-inflammatory drugs were the most common therapy prescribed to patients (87%). “Cardiac MRI was often used for diagnostic purposes and also for possible prognostic purposes,” Oster and colleagues noted. “Supportive care was a mainstay of treatment, with specific cardiac or intensive care therapies as indicated by the patient’s clinical status.”

The cases of myocarditis reported after COVID-19 mRNA vaccination were generally diagnosed within days of patients’ vaccination—contrasted to typical viral myocarditis cases, which can often have “indolent courses with symptoms sometimes present for weeks to months after a trigger if the cause is ever identified.”

Investigators also noted that the major presenting symptoms of myocarditis in vaccinated patients seemed to resolve faster than typical viral cases: “Even though almost all individuals with cases of myocarditis were hospitalized and clinically monitored, they typically experienced symptomatic recovery after receiving only pain management.”

Though long-term effects and recovery of myocarditis cases in COVID-19 vaccinated persons is still under assessment, investigators did observe in these acute cases that 96% of patients were hospitalized and 87% had resolution of presenting symptoms by their discharge. No deaths were observed among hospitalized patients.

The CDC previously began follow-up surveillance in vaccinated adolescents and young adults in order to better interpret health and functional status, including cardiac outcomes, 3-6 months following probably and confirmed cases of myocarditis. Oster and colleagues emphasized the American Heart Association (AHA) and American College of Cardiology (ACC) guidelines advising that such patients should refrain from competitive sports for 3-6 months and that documentation of normal electrocardiogram result, ambulatory rhythm monitoring, and an exercise test should be completed prior to their resumption.

Further doses of mRNA-based COVID-19 vaccines should be deferred, but may be considered in select circumstances,” they wrote.

The study, “Myocarditis Cases Reported After mRNA-Based COVID-19 Vaccination in the US From December 2020 to August 2021,” was published online in JAMA.