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A study in a relatively diverse population reaffirmed acute myocarditis as a rare and fairly mild event affecting young men after COVID-19 mRNA vaccination.

There were 15 confirmed cases of myocarditis over a 10-day observation window among the more than 2.3 million Kaiser Permanente Southern California (KPSC) adult members vaccinated against SARS-CoV-2 from December 2020 to July 2021. The two myocarditis cases after a first vaccine dose translated into an incidence of 0.8 cases per 1 million first doses; the 13 cases after a second dose represented 5.8 cases per 1 million second doses.

All 15 individuals were men, with a median age of 25 years, and none had cardiac disease before they were hospitalized for myocarditis. Patients were roughly split between those who had received the Pfizer and Moderna vaccines, reported Ming-Sum Lee, MD, PhD, of Kaiser Permanente Los Angeles Medical Center, and colleagues in .

All but one had complained of chest pain in the days immediately after COVID vaccination. All cases of acute myocarditis, independently adjudicated by at least two cardiologists, resolved with conservative management. No one was admitted to the ICU or required readmission after discharge.

Notably, of the people vaccinated in the KPSC system, 37.8% were Hispanic, 31.2% were white, 14.3% were Asian, and 6.7% were Black. Women accounted for 54.0% of the total cohort, and median age was 49 years (35.7% of individuals were younger than 40 years). Of the 15 men who developed myocarditis, nine were white, four were Hispanic, one was Asian, and one was of unknown ethnicity.

"This is an important study since it further provides evidence for a rare but clinically important adverse event with mRNA vaccination. The diverse population helps generalize the data to the larger population," wrote Amanda Verma, MD, and Kory Lavine, MD, PhD, both of Washington University School of Medicine in St. Louis, in a joint commentary provided to MedPage Today.

"The vaccinated population was diverse, [and] the myocarditis cases were few," commented Biykem Bozkurt, MD, PhD, of Baylor College of Medicine in Houston. "Though the numbers are too small to make a conclusion, demographics of myocarditis cases did not reflect a higher incidence of myocarditis among underrepresented race/ethnicity patients," she wrote in an email.

The incidence rate ratio (IRR) for myocarditis was found to be statistically no greater after the first dose of COVID vaccination than for unexposed individuals during the study period; it was the second dose that put vaccinated individuals over the edge in excess risk (IRR 2.7, 95% CI 1.4-4.8), Lee and colleagues noted.

These findings were consistent with sensitivity analyses using vaccinated people as their own controls based on the incidence of myocarditis during a 10-day period a year prior to vaccination.

Bozkurt concluded that the KPSC report supports prior studies suggesting a small risk for myocarditis after mRNA vaccination, predominantly in young men.

Moreover, the risk of post-vaccination myocarditis is "small when weighed with the morbidity and mortality of COVID-19 infection, in which up to 28% of hospitalized patients showed signs of myocardial injury," according to an by two members of the JAMA Internal Medicine team.

"Randomized clinical trials show that COVID-19 mRNA vaccines represent a safe and effective method of preventing infection; the identification of rare myocarditis does not change clinical decision-making. However, it would be worthwhile to identify the mechanism of cardiac injury from vaccines. In addition, we anticipate seeing more cases of myocarditis, as vaccination [is authorized] for teenage males aged 12 to 16 years," wrote editorial fellow Vinay Guduguntla, MD, of the University of California San Francisco, and deputy editor Mitchell Katz, MD, of NYC Health + Hospitals.

The large population-based cohort study of vaccinated KPSC members showed that 93.5% had completed their two doses.

Myocarditis events were collected from reports from clinicians to an institutional committee and from records on hospitalizations within 10 days of vaccine administration.

Verma and Lavine, who were not involved with the study, pointed out that all patients with myocarditis had been hospitalized in this report, whereas prior reports suggested that vaccine-associated cases are usually mild and resolve quickly without hospitalization.

"This raises the question as to whether the rates of outpatient reporting were similar to inpatient reporting. Furthermore, it's difficult to understand how the diagnosis of myocarditis was made since not all patients seem to have had imaging and other causes of myocarditis were not discussed in these patients," the duo noted.

Lee and colleagues acknowledged the possibility of subclinical cases being underdiagnosed, along with other study limitations, such as the observational design and short follow-up time.

"No relationship between COVID-19 mRNA vaccination and postvaccination myocarditis can [be] established given the observational nature of this study," they cautioned.

"Nonetheless, studies such as this one continue to observe a fortunately rare but clinically significant adverse effect of COVID-19 mRNA vaccines that is important for the medical community to recognize," wrote Verma and Lavine.

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