Small Myocarditis Risk Now Seen for Adenovirus-based COVID-19 Vaccine

Patrice Wendling

December 16, 2021

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.

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 (IRRs) for myocarditis 1 to 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," Mills, from the University of Edinburgh, United Kingdom, told theheart.org | Medscape Cardiology. Mills is a coauthor on the study, published December 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 SARS-CoV-2 positive 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

  • 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," 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, Texas, told theheart.org | Medscape Cardiology.

"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," 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 UT Southwestern Medical Center in Dallas, Texas, and co-chair 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 one or two per million," he told theheart.org | Medscape Cardiology.

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 healthcare database of 38.6 million people, aged 16 years or older, vaccinated from December 1, 2020, to August 24, 2021.

The number of people admitted to the hospital or who died during the study period was 1615 for myocarditis, 1574 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 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," Mills said during a press briefing.

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 note, 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," 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 to 20 and 20 to 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," 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."

Mills reports having no relevant disclosures. Bozkurt reports 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. De Lemos reported having no relevant conflicts of interest.

Nat Med. Published online December 14, 2021. Full text

Follow Patrice Wendling on Twitter: @pwendl. For more from theheart.org | Medscape Cardiology, join us on Twitter and Facebook.

Comments

3090D553-9492-4563-8681-AD288FA52ACE
Comments on Medscape are moderated and should be professional in tone and on topic. You must declare any conflicts of interest related to your comments and responses. Please see our Commenting Guide for further information. We reserve the right to remove posts at our sole discretion.

processing....