COVID-19 mRNA vaccines may be 'new trigger' for heart inflammation, CDC group says, but benefit outweighs risk

A U.S. Centers for Disease Control and Prevention (CDC) advisory group now says reports of heart inflammation in people given an mRNA-based COVID-19 vaccine are likely linked to the vaccine, but that the benefits of vaccination still outweigh any risks.

In a Wednesday presentation, the CDC's Advisory Committee on Immunization Practices (ACIP) noted that early data from its database shows a rate of 4.4 reported cases of heart inflammation per million first doses given of any mRNA vaccine in the 21 days following vaccination.

That rate jumped to 12.6 reported cases per million second doses.

The reports were greater than anticipated, particularly after dose two in younger age groups, with early data on cases among 12- to 39-year-olds suggesting the case rate is higher in males than females.

During the presentation, Dr. Matt Daly — chair of the ACIP COVID-19 vaccines working group — emphasized the strength of the U.S. vaccine safety monitoring system.

"However, nothing in life is absolutely risk free," he said. "And we know from decades of experience with other vaccines that rare but potentially serious events can occur following vaccination."

The presentation focused on several conditions being reported post-vaccination, including myocarditis, referring to inflammation of the myocardium, which is the heart muscle, and pericarditis, meaning inflammation of the pericardium, which is the lining around the heart.

Myopericarditis is a condition where both myocarditis and pericarditis are present.

Mechanism still unclear

The CDC has been investigating cases of heart inflammation mainly in young men for several months, and said earlier this month that it was still evaluating the risk from the condition and did not confirm a causal relationship between the vaccines and the heart issue.

On Wednesday, the CDC's working group said available data does suggest a likely link between myocarditis and mRNA vaccination in adolescents and young adults.

"We do not know potential mechanisms yet," Dr. Matthew Oster, a member of the CDC's COVID-19 vaccine task force, said during the presentation.

Myocarditis is rare but not a new disease, he said, with the findings suggesting mRNA vaccines "may be a new trigger."

The CDC's available outcome data also showed patients suffering from post-vaccination heart inflammation generally do recover from their symptoms, which can include chest pain.

Many patients have been hospitalized, "usually for short duration," the presentation slides note, which noted that there's no long-term data available yet.

"Currently, the benefits still clearly outweigh the risks for COVID-19 vaccination in adolescents and young adults," reads a slide from the presentation.

WATCH | Reports of post-vaccination heart inflammation among small number of children:

Reports of heart inflammation among small number of children that got Pfizer vaccine

3 years ago

Duration 2:03

A small number of children who received the Pfizer-BioNTech vaccine have reported swelling in the heart. Researchers and officials are tracking the phenomenon closely, but so far they've found no cause for alarm.

Condition reported in various countries, including Canada

The findings follow reports of post-vaccination heart inflammation in various countries including the U.S., Italy, Israel and Canada.

In Israel, 275 cases of myocarditis were reported between December 2020 and May 2021 among more than five million vaccinated people, the country's health ministry said in early June.

Most patients who experienced heart inflammation spent no more than four days in hospital and 95 per cent of the cases were classified as mild.

The latest-available Canadian vaccine safety data shows there have been at least 53 cases of myocarditis or pericarditis reported to Health Canada or the Public Health Agency of Canada as of June 11, with the majority occurring following vaccination with mRNA vaccines made by Pfizer-BioNTech and Moderna.

"To date, no clear association has been established between myocarditis/pericarditis and COVID-19 vaccines," reads the most recent federal vaccine safety update.

While the rates in Canada aren't yet above the expected levels, immunologist and McMaster University associate professor Matthew Miller said that's likely because many younger Canadians have only received their first dose.

"As those individuals begin to get second doses, I think it would be reasonable to expect to start to see a signal in that cohort," he said. "Countries that are further ahead have been able to pick this up much earlier, based on more people in that age group having more of their second doses."

Pfizer, whose vaccine has been authorized for use in Americans as young as 12, previously said it had not observed a higher rate of heart inflammation than would normally be expected in the general population.

Moderna also said it could not identify a causal association with the heart inflammation cases and its vaccine.