Sex- and age-stratified rates
In a separate analysis of 5,442,696 people given a first dose of the Pfizer BNT162b2 vaccine and 5,125,635 given a second dose, there were 142 cases of myocarditis with onset 21 days after dose 1 and 30 days after dose 2. Of those cases, 136 were documented as “definite or probable” in an Israeli Ministry of Health database that covered up to the end of May 2021.
There were also 40 cases among vaccinated people seen after the 30-day window, which were considered not related to the vaccination, and 101 cases among unvaccinated people; of the latter, 29 had confirmed diagnoses of COVID-19.
Of the 136 people with definite or probable cases, the myocarditis was “generally mild” in 129 and usually resolved on its own, notes the report on the study, published in the New England Journal of Medicine, with lead author Dror Mevorach, MD, Hadassah-Hebrew University Medical Center, Jerusalem.
The estimated myocarditis incidence after a second such vaccine dose across the entire Israeli population, based on the current study, was about one per 26,000 males and one per 218,000 females, the group writes. Those figures compare with one case per 10,857 among “the general unvaccinated population.”
Again, the risk was concentrated among younger men and male adolescents. In an analysis limited to vaccinated people aged 16-19 years, myocarditis in the 21 days after a second mRNA injection was seen in about one of 6,637 males and one of 99,853 females, the group reported.
The standardized incidence ratio of 5.34 (95% CI, 4.48-6.40) after a second injection, across all groups, “was driven mostly by the diagnosis of myocarditis in younger male recipients.” Among that male subgroup, the ratios by age group were 13.60 (95% CI, 9.30-19.20) for 16-19 years, 8.53 (95% CI, 5.57-12.50) for 20-24 years, and 6.96 (95% CI, 4.25-10.75) for 25-29 years.
Among people who received a second injection, compared with unvaccinated people, the 30-day rate ratio was 2.35 (95% CI, 1.10-5.02). Again, the effect was concentrated in males aged 16-19 years. Among them, the myocarditis rate ratios in the 30 days after a second mRNA vaccine injection were 8.96 (95% CI, 4.50-17.83) for the 16-19 years group, 6.13 (95% CI, 3.16-11.88) for the 20-24 group, and 3.58 (95% CI, 1.82-7.01) for 25-29 years.
Most of the patients with myocarditis showed “significant clinical improvement,” with a mean hospitalization time of only 3-4 days, the report notes. Treatment consisted of nonsteroidal anti-inflammatory drugs “with or without colchicine for presumed pericardial inflammation.”
However, seven patients (4.9%) developed important complications, including left-ventricular dysfunction, ventricular arrhythmias, and heart failure. Among them was a 22-year-old patient who died of fulminant myocarditis within 24 hours of diagnosis, the group wrote.
From an Israeli health care organization
Published by the same journal as the study by Dr. Menvorach and associates, an analysis of a separate database showed largely consistent findings among patients in the largest of Israel’s four health care organizations charged by the government to administer health services.
The report, with authors led by Guy Witberg, MD, Rabin Medical Center, Petah Tikva, Israel, focused on members of the health care organization aged 16 years or older who had received at least one Pfizer mRNA vaccine dose by the end of May 2021.
The cohorts from the two separate reports surely overlap substantially, as the Ministry of Health analysis from Dr. Mevorach and colleagues derived from a nationwide database, and – as Dr. Witberg and associates wrote – the health care organization providing their data covers 52% of the Israeli population.
Of 2,558,421 vaccinated people in the analysis, of whom 94% received two doses, 54 developed confirmed myocarditis in the 42 days after the first dose. Their median age was 27 years (interquartile range, 21-35 years) and all but three (94%) were male. Of those 54 cases, 41 were considered mild and 12 intermediate in severity, and one was fulminant with the patient in cardiogenic shock, the group writes. In addition, nonsustained ventricular tachycardia and atrial fibrillation developed in 5% and 3% of cases, respectively.
The estimated myocarditis incidence in the 42 days after administration of at least one mRNA vaccine dose was 2.13 per 100,000 vaccinated people. In that group, Dr. Witberg and colleagues note, the corresponding incidences per 100,000 were 4.12 and 0.23 for males and females, respectively.
Also in the current report, incidences per 100,000 vaccinated people aged 16-29 years, by sex, included 5.49 (95% CI, 3.59-7.39) overall, and 10.69 (95% CI, 6.93-14.46) for males (the highest rate in the report).
There was only one case in a female aged 16-29 years, and two cases in females 30 years or older.
Of note, some authors of the current study are also authors on the high-profile report from Noam Barda, MD, and colleagues published in the New England Journal of Medicine, that used the same database to arrive at an mRNA-vaccine-related incidence of myocarditis of 2.7 per 100,000. Eligibility criteria and follow-up time were different in that report, as were case ascertainment criteria.
The myocarditis risk associated with the two mRNA vaccines is small compared with “the morbidity and mortality of COVID-19 infection, in which up to 28% of hospitalized patients showed signs of myocardial injury,” wrote Vinay Guduguntla, MD, University of California, San Francisco, and Mitchell H. Katz, MD, NYC Health + Hospitals, New York, in an editorial accompanying the report from Dr. Lee and associates.
“Randomized clinical trials show that COVID-19 mRNA vaccines represent a safe and effective method of preventing infection,” they stated. “The identification of rare myocarditis does not change clinical decision-making.”
Dr. Bozkurt, who is immediate past president of the Heart Failure Society of America, has disclosed consulting for Bayer and scPharmaceuticals and serving on a clinical events committee for a trial supported by Abbott Pharmaceuticals and on a data and safety monitoring board for a trial supported by Liva Nova Pharmaceuticals. Dr. Lee and the report’s other authors had no disclosures. Dr. Mevorach discloses consulting for Enlivex Therapeutics; disclosures for the other authors are available at NEJM.org. Dr. Witberg said he has no interests to disclose; disclosures for the other authors are available at NEJM.org. Dr. Guduguntla is an editorial fellow and Dr. Katz a deputy editor at JAMA Internal Medicine; neither had disclosures.
A version of this article first appeared on Medscape.com.