Study: Myocarditis risk 2 to 3 times higher from Moderna than Pfizer COVID-19 vaccine

The incidence of myocarditis -- inflammation of the heart muscle -- is two- to threefold higher after a second dose of the Moderna Spikevax COVID-19 vaccine compared to the Pfizer BioNTech COVID-19 vaccine, a new study says. File photo by Debbie Hill/UPI
The incidence of myocarditis -- inflammation of the heart muscle -- is two- to threefold higher after a second dose of the Moderna Spikevax COVID-19 vaccine compared to the Pfizer BioNTech COVID-19 vaccine, a new study says. File photo by Debbie Hill/UPI | License Photo

Nov. 7 (UPI) -- The incidence of myocarditis -- inflammation of the heart muscle -- is two- to threefold higher after a second dose of the Moderna Spikevax COVID-19 vaccine than the Pfizer BioNTech COVID-19 vaccine, a study released Monday says.

But Moderna tells UPI the benefits of its vaccine "significantly outweigh" the risks.


Males under age 40 who received the Moderna vaccine were shown to have the highest rates of myocarditis, and the researchers said the study's findings support the idea of recommending specific vaccines for certain populations to maximize benefits and minimize adverse events.

But the scientists underscored that cases of heart inflammation as a serious side effect from either mRNA vaccine are rare overall -- also stressed by the Centers for Disease Control and Prevention, which said it is actively monitoring the situation, and by the manufacturer itself.


The most common symptoms of myocarditis are chest pain, fever, fatigue, shortness of breath, and a rapid or irregular pulse, says the National Institutes of Health. Heart inflammation can lead to serious complications, including heart failure, shock or death.

Moderna, in an emailed statement provided by spokesman Luke Mircea-Willats, said its "mRNA-1273 has been administered to hundreds of millions of people worldwide and has been shown to be effective against both the original strain of the virus and its major variants.

"Regulatory agencies around the world have stated that the benefits of COVID-19 mRNA vaccines significantly outweigh the risk across all age groups. Vaccination against COVID-19 continues to be a critical tool in overcoming the impacts of the global pandemic."

According to Moderna, myocarditis is a known, though "very rare" risk associated with COVID-19 mRNA vaccines and, "when it does occur, cases are generally mild and resolve after a few days with treatment and rest."

And, "at a population level," current evidence indicates the risk of myocarditis after COVID-19 infection is much higher than after COVID-19 vaccination, Moderna said.

Moderna stressed that patients' health and ensuring the safety of its vaccines is its top priority, noting it "shares all adverse events data with regulators and has a robust pharmacovigilance function, ensuring any adverse events are recorded and shared with the regulator in line with local regulations."


Results from the new study, which appeared in the Journal of the American College of Cardiology, "generally align" with findings from previous studies into indirect comparison of COVID19 mRNA vaccine product safety, the researchers said in their paper.

Epidemiologist Naveed Janjua, the study's lead author, said the findings "have implications for strategizing the rollout of mRNA vaccines, which should also consider the self-limiting and mild nature of most myocarditis events, benefits provided by vaccination, higher effectiveness of the Moderna vaccine against infection and hospitalization [found in prior studies], and the apparent higher risk of myocarditis following COVID-19 infection than with mRNA vaccination."

Janjua, who is executive director of Data and Analytic Services at the British Columbia Centre for Disease Control, made his remarks in a news release.

In an accompanying editorial, Dr. Guy Witberg, a cardiologist at Rabin Medical Center in Petah-Tikva, Israel, wrote that the study is reassuring for vaccine safety since it provides further data that myocarditis is a very rare adverse event after both vaccines.

He also called it "an important step" toward a "personalized and tailored approach to vaccination."

Of the two mRNA COVID-19 vaccines approved for use, more than 52 million doses of Pfizer BioNTech (BNT162b2) and 22 million doses of Moderna Spikevax (mRNA-1273) had been administered as of March 20 in Canada, where the study was performed, the researchers noted.


According to them, few studies have directly compared the safety of the two mRNA COVID-19 vaccines, which the scientists said "differ in important ways that could impact safety."

So, the population-based analysis sought to compare the risk of myocarditis, pericarditis, or inflammation of the heart's outer lining, and myopericarditis -- a combination of the two conditions -- between the Pfizer and Moderna COVID-19 vaccines.

The study's participants were 18 years old or older and had gotten two primary doses of either Pfizer or Moderna vaccine in British Columbia, Canada, with the second dose between Jan. 1, 2021, and Sept. 9, 2021.

Using data from roughly 3 million people in the British Columbia COVID-19 Cohort, a population-based cohort study, the investigators looking for a diagnosis of myocarditis, pericarditis or myopericarditis during a hospitalization or emergency department visit within 21 days of the second dose of an mRNA COVID-19 vaccine.

People with a history of myocarditis or pericarditis within one year prior to the second vaccine dose were excluded.

Overall, more than 2.2 million second Pfizer doses were given and more than 870,000 Moderna doses. Within 21 days of the second dose, there were a total of 59 myocarditis cases, 21 after Pfizer and 31 after Moderna; and there were a total of 41 pericarditis cases: 21 after Pfizer and 20 after Moderna.


Researchers analyzed rates per million doses and the rate was 35.6 cases per million for Moderna and 12.6 cases per million for Pfizer, almost threefold higher.

Rates of myocarditis and pericarditis were higher with the Moderna vaccine in both males and females between the ages of 18 and 39, with the highest per million rates in males ages 18-29 after a second dose of Moderna.

According to the researchers, one limitation of the study is that it was observational, which limits the ability to determine causality between vaccination and myocarditis or pericarditis.

Yet, the study was designed to limit the time between vaccine dose and myocarditis/pericarditis diagnosis.

Another limitation is that the study relied on hospital and emergency department visit data, so the researchers may have missed some less severe cases, the scientists said.

The study follows on the heels of Moderna's third-quarter earnings report released Nov. 3. The 2022 sales forecast for its COVID-19 vaccine was lowered to between $18 billion and $19 billion in revenue, down from $21 billion.

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