Large-scale data
The investigators examined the rates of hospital admission or death from myocarditis, pericarditis, and cardiac arrhythmia in the 28 days following SARS-CoV-2 vaccination or infection by linking the English National Immunisation Database of COVID-19 vaccination with a national patient-level health care database of 38.6 million people, aged 16 years or older, vaccinated from Dec.1, 2020, to Aug. 24, 2021.
The number of people admitted to the hospital or who died during the study period was 1,615 for myocarditis, 1,574 for pericarditis, and 385,508 for cardiac arrhythmia.
There was no evidence of an increased risk for pericarditis or cardiac arrhythmia following vaccination, except for arrhythmia in the 28 days following a second dose of the Moderna vaccine (IRR, 1.46).
In contrast, the risk was increased for pericarditis (IRR, 2.79) and cardiac arrhythmia (IRR, 5.35) in the 28 days following a positive SARS-CoV-2 test result.
Although the scale of the analysis allows for more precise estimates than what’s been possible in smaller data sets, there is the challenge of diagnosing COVID-19 from billing codes and the potential for ascertainment bias, noted Dr. de Lemos.
“Having said that, I think it’s a really important study, because it’s the first study to put the incidence in context in the same general population the risks of myocarditis with various vaccines and with COVID-19,” he said.
“That’s really important and provides a lot of reassurance for those who are trying to balance the risks and benefits of vaccination.”
Analyses by sex and age
A subgroup analysis by age showed increased risks for myocarditis with the mRNA vaccines only in those younger than 40, whereas no association was found with the Oxford adenovirus vaccine.
“We’re not seeing any signal here that would make us change the recommendation for vaccination in children as a consequence of this risk,” Dr. Mills said during a press briefing.
Dr. Bozkurt pointed out, however, that the estimated excess in myocarditis events following a second dose of the Moderna vaccine in these younger adults reportedly exceeded that for SARS-CoV-2 infection (15 per million vs. 10 per million).
“For that age group, it’s concerning and needs further clarification. This hasn’t been seen before,” she said.
The average age was 39 years for those receiving two doses of the Moderna vaccine and 55 for recipients of the Pfizer and Oxford vaccines. The Moderna vaccine wasn’t rolled out until April 2021 in the United Kingdom, the authors noted, so the number of patients who received this vaccine is lower.
Although reports have suggested young males are at greater risk for myocarditis after vaccination, an analysis by sex found that women had an increased risk for myocarditis after a first dose of the AstraZeneca (IRR, 1.40) and Pfizer (IRR, 1.54) vaccines and following a positive COVID-19 test result (IRR, 11.00).
“Women being at increased risk is rather a new message,” Dr. Bozkurt said. “But the incidence rate ratios are being compared against the unvaccinated, so when you see the increase in women, it doesn’t mean it’s increased against men. It would be helpful for sex-specific incidence rate ratios to be reported for younger age subgroups, such as ages 16-20 and 20-30, to determine whether there’s an increased risk for males compared to females at younger ages.”
Age and sex differences are huge questions, but “I think we’ll learn a lot about myocarditis in general from what is going to be an explosion of research into the vaccine-associated causes,” Dr. de Lemos said.
“That will help us understand myocarditis more broadly and prepare us for the next generation of vaccines, which inevitably will be mRNA based.”
Dr. Mills reported having no relevant disclosures. Dr. Bozkurt reported 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. De Lemos reported having no relevant conflicts of interest.
A version of this article first appeared on Medscape.com.