From the Journals

Small myocarditis risk now seen for adenovirus-based COVID-19 vaccine



The first large population study to investigate the association between different COVID-19 vaccines types and cardiac effects and adverse events shows a small increase in the risk for acute myocarditis with both the mRNA-based vaccines and – in what may a first in the literature – an adenovirus-vector vaccine.

Young African American girl getting COVID-19 vaccine Ivan Pantic/Getty Images

The excess risk was seen following the first dose of the ChAdOc1 (AstraZeneca/Oxford), the adenovirus-based vaccine, and the mRNA-based BNT162b2 (Pfizer/BioNTech). It was observed after first and second doses of the mRNA-1273 (Moderna) vaccine.

The incidence rate ratios for myocarditis 1-7 days after the first AstraZeneca, Pfizer, and Moderna injections were 1.76, 1.45, and 8.38, respectively, and 23.1 after the second dose of the Moderna vaccine.

“There’s a bit more uncertainty and worry about mRNA vaccines because it’s quite a new vector for vaccination and, therefore, there’s been more focus on the potential side effects,” said Nicholas Mills, MD.

“But it doesn’t surprise me the signal is present for all types of vaccines because they’re designed to generate a systemic immune response and that is, unfortunately, where you can cause small risks for immune-mediated illnesses like myocarditis,” Dr. Mills, from the University of Edinburgh, told this news organization. Dr. Mills is a coauthor on the study, published Dec. 14 in Nature Medicine.

To put the risks in context, the group estimated between 1 and 10 additional myocarditis hospitalizations or deaths per 1 million people vaccinated, but 40 excess myocarditis events per million following a positive SARS-CoV-2 test result.

As reported, rates of excess myocarditis events associated with a first dose were 2 per million injections of the AstraZeneca vaccine, 1 per million for the Pfizer vaccine, and 6 per million with the Moderna vaccine.

Following a second dose, there were 10 additional myocarditis events per million people receiving the Moderna vaccine and none among recipients of the AstraZeneca or Pfizer vaccines.

“It was particularly seen within the first 7 days of the first dose, which is very consistent with what we see in people who have viral myocarditis,” Dr. Mills said. “So it looks like a real signal but it’s very small.”

The results are in line with previous studies of the Pfizer vaccine in Israel and studies of the Moderna vaccine in the United States, Biykem Bozkurt, MD, PhD, professor of medicine at Baylor College of Medicine, Houston, told this news organization.

“What this paper does is confirm that cardiovascular complications – and they are only looking at a small component of those cardiovascular complications – are markedly higher with the COVID-19 infection than with the vaccines,” she said.

It also adds a new twist to the search for the mechanisms of myocarditis, which has focused on the immunogenicity of the RNA in the Pfizer and Moderna vaccines but also hypothesized that molecular mimicry between the SARS-CoV-2 spike glycoprotein and cell antigens, antibody production against cardiac proteins, and testosterone may play a role.

“But now it doesn’t look like the risk is solely confined to the mRNA vaccine platform because it’s also happening with the adenovirus,” Dr. Bozkurt said. “The mechanisms require future experimental and clinical research and we’ll need more granular data with cohorts that are closely followed up as well as subclinical follow-up.”

James de Lemos, MD, professor of medicine at the University of Texas Southwestern Medical Center, Dallas, and cochair of the American Heart Association’s COVID-19 CVD Registry, said he was also not surprised by a myocarditis signal with AstraZeneca’s adenovirus vaccine.

“Looking at relative risks has biological implications, but the clinical and public health implications are that the absolute risk with the adenovirus is trivial. And you see that with their estimations of absolute risk where it’s literally sort of a needle in the haystack of 1 or 2 per million,” he said in an interview.


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