Myocarditis after COVID-19 vaccination infrequent; signals observed in young men
In two studies from Israel published in The New England Journal of Medicine, incidence of myocarditis after COVID-19 vaccination remained low; however, almost all documented cases were in young men.
In Israel, the nationwide campaign to administer the BNT162b2 messenger RNA (mRNA; Pfizer-BioNTech) vaccine began Dec. 20, and by May 5, 58.2% of the Israeli population had received at least one dose and 54.6% were fully vaccinated.
On May 27, the CDC issued a statement on the possible association between COVID-19 vaccination and risk for myocarditis for both the BNT162b2 vaccine and the mRNA-1273 vaccine (Moderna), and by June 2, the Israeli Ministry of Health reported 148 cases of myocarditis.
The present studies evaluated the incidence of myocarditis after administration of the BNT162b2 mRNA vaccine and described the clinical course and disease severity.
Health services database
One retrospective analysis utilized the Clalit Health Services database to identify cases of myocarditis among patients who received at least one dose of the BNT162b2 mRNA vaccine. The myocarditis diagnosis was adjudicated by cardiologists using the CDC’s case definition.
Of 2.5 million vaccinated individuals aged at least 16 years, 54 met the criteria for myocarditis.
The estimated incidence of myocarditis among individuals who received at least one dose of the BNT162b2 mRNA vaccine was 2.13 per 100,000 persons (95% CI, 1.56-2.7) with the highest incidence reported among young men aged 16 to 29 years (10.69 cases per 100,000 persons; 95% CI, 6.93-14.46).
According to Guy Witberg, MD, interventional cardiologist at the Rabin Medical Centre in Petah-Tikva, Israel, and colleagues, 76% of the myocarditis cases were described as mild, 22% were described as intermediate and one case was associated with cardiogenic shock.
After a median 83 days of follow-up after myocarditis onset, one patient was readmitted to the hospital, and another died of an unknown cause after discharge.
According to the study, 14 patients had left ventricular dysfunction on echocardiography at hospital admission, of whom 10 continued to experience dysfunction at discharge. Of these 10 patients, five had normal heart function on subsequent testing.
“Although we cannot directly compare the incidence of myocarditis after vaccination in our study with the incidence in other studies, our data may provide points of reference,” Witberg and colleagues wrote. “On the basis of data from the Vaccine Adverse Events Reporting System, the CDC has estimated that the incidence of myocarditis after any COVID-19 vaccination is 0.48 cases per 100,000 overall and 1.2 cases per 100,000 among vaccine recipients between the ages of 18 and 29 years. These estimates are lower than those in our study, possibly due to different methods that were used to identify cases (passive reporting to the CDC vs. electronic health records in our health care organization).”
Israeli Ministry of Health data
In another retrospective study, Dror Mevorach, MD, professor in the department of medicine at Hadassah Hebrew University Medical Center in Jerusalem, and colleagues evaluated data obtained from active surveillance by the Israeli Ministry of Health from Dec. 20 to May 31 on all cases of myocarditis.
Researchers categorized the information using the Brighton Collaboration definition.
Researchers assessed the risk difference for myocarditis by comparing incidence after the first and second vaccine dose of the BNT162b2 mRNA vaccine; calculated the incidence ratio of myocarditis within 21 days after the first dose and 30 days after the second dose; and calculated the rate ratio of myocarditis within 30 days among individuals who received the second dose compared with unvaccinated people.
A total of 283 individuals experienced symptoms of myocarditis, of which 142 occurred after receipt of the BNT162b2 mRNA vaccine; of those, 136 were determined definitive or probable. The clinical presentation of myocarditis was mild in 95% of the cases.
Researchers reported that the overall risk difference between the first and second doses of the vaccine was 1.76 per 100,000 persons (95% CI, 1.33-2.19). Mevorach and colleagues observed the largest risk difference among young men aged 16 to 19 years (13.73 per 100,000 persons; 95% CI, 8.11-19.46).
Compared with expectations based on historical data, the incidence ratio of myocarditis after COVID-19 vaccination was 5.34 (95% CI, 4.48-6.4) and was highest after the second dose among young men aged 16 to 19 years (13.6; 95% CI, 9.3-19.2).
Moreover, the rate ratio of myocarditis at 30 days after the second vaccine dose was 2.35 compared with unvaccinated individuals (95% CI, 1.1-5.02), and the risk was highest for young men aged 16 to 19 years (RR = 8.96; 95% CI, 4.5-17.83), with a ratio of 1 in 6,637.
There was one fatal case of fulminant myocarditis, according to the researchers.
“The incidence of myocarditis, although low, increased after the receipt of the BNT162b2 vaccine, particularly after the second dose among young male recipients,” Mevorach and colleagues wrote.
“In our study, the rate of myocarditis in the general unvaccinated population was 1 per 10,857 and can be compared with findings indicating that myocarditis was more common after SARS-CoV-2 infection than after vaccination,” they wrote.