Myocarditis is a rare but serious side effect of COVID-19 vaccines that occurs most often in people under age 21. The latest research offers some reassurance that most young people recover quickly from it.
Myocarditis – inflammation of the heart muscle – can weaken the organ and interfere with its electrical system, which keeps the heart pumping regularly. It occurs most often after an infection or inflammation caused by a virus, according to the American Heart Association.
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Previous research has shown that the smallpox vaccine triggered a similar inflammatory response in the heart. The cases related to COVID-19 vaccines mostly occur in male adolescents and young adults.
The latest study, published by the American Heart Association, examined 139 teenagers and young adults who developed myocarditis within 30 days of vaccination. The vast majority had received one of the COVID-19 vaccines produced by Pfizer or Moderna and developed myocarditis after their second doses.
The researchers reported that 18.7% of them required intensive care, but there were not any deaths. Most patients were hospitalized for 2-3 days.
Cardiac MRIs showed evidence of inflammation or injury to the heart muscle in most patients. About 18.7% had at least mildly decreased left ventricular function, but heart function returned to normal in all of the patients who came back for a follow-up evaluation.
The most common symptoms were chest pain, fever and shortness of breath. Symptoms generally developed within two days of vaccination.
Dr. Donald. M. Lloyd-Jones, president of the American Heart Association, who was not involved in the study, said that research continues to find vaccine-related cases of myocarditis to be rare and mostly mild.
"Overwhelmingly, data continue to indicate that the benefits of COVID-19 vaccination – 91% effective at preventing complications of severe COVID-19 infection including hospitalization and death – far exceed the very rare risks of adverse events, including myocarditis," he said.
Incidence rates of vaccine-related myocarditis vary slightly among studies. An Israeli study published recently in the New England Journal of Medicine also found incidence to be rare and mostly mild or moderate in severity. The researchers found an estimated incidence of 2.13 cases per 100,000 persons.
An August study published in JAMA Network reported higher incidence rates across 40 hospitals in Washington, Oregon, Montana and Los Angeles County, California.
Further studies are needed to understand the long-term outcomes of patients who develop myocarditis after COVID-19 vaccination, researchers said. More data also is needed on the possible risk factors and the underlying mechanisms behind this rare side effect.
The latest study was based on data from 26 pediatric medical centers in the United States and Canada. All vaccine-related myocarditis cases involved in the study occurred before July 4. The median age of the participants was 15.8 years. Most were were and male.