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Several studies show that adolescents and young males have a higher risk of myocarditis after COVID-19 vaccination, but this condition is more common after coronavirus infection. Luza studios / Getty Images
  • Several studies have found that the risk of heart inflammation after mRNA COVID-19 vaccination is real, but it’s fairly mild and extremely rare.
  • Most studies show that adolescents and young males have a higher risk, particularly after the second dose.
  • The risk of myocarditis after coronavirus infection has been found to be higher than after vaccination.

The risk of heart inflammation after mRNA COVID-19 vaccination is a rare and fairly mild event, research shows.

Adolescents and young males have a higher risk, particularly after the second dose. One study also found that the risk was lower after a booster dose.

Myocarditis can also occur after coronavirus infection, with rates higher than after COVID-19 vaccination.

Here is a summary of several recent studies.

A study of over 23 million people aged 12 years or older in four Nordic countries examined the risk of myocarditis after receiving an mRNA COVID-19 vaccine.

Researchers found that the risk of heart inflammation was highest in males aged 16 to 24 years after the second dose.

The risk was also higher in this group after a second dose of the Moderna vaccine compared to the Pfizer-BioNTech vaccine.

In addition, among all age groups, the risk of myocarditis was higher after the second dose.

The study was published April 20, 2022, in JAMA Cardiology.

In this research, results from four separate studies — carried out in Denmark, Finland, Norway, and Sweden — were combined and reanalyzed in what’s known as a meta-analysis.

A group of researchers from Singapore reviewed 22 studies that looked at the inflammation of the heart or the tissue around the heart — myocarditis or pericarditis — after vaccination.

These studies included several types of vaccines, such as COVID-19, smallpox, and seasonal influenza.

The rate of myocarditis or pericarditis was higher after smallpox vaccination compared to the COVID-19 vaccination. Heart inflammation was similar after receipt of the influenza vaccine and COVID-19 vaccine.

Studies looking at the COVID-19 vaccine found that the risk of heart inflammation was higher in males, people younger than 30 years old, after receiving an mRNA vaccine and after the second dose.

The study was published April 11, 2022, in The Lancet Respiratory Medicine.

Researchers from the Centers for Disease Control and Prevention (CDC) reviewed around 3,400 reports of side effects experienced by 12- to 17-year-olds after a second dose or a booster dose of the Pfizer-BioNTech COVID-19 vaccine.

Over 2.8 million U.S. adolescents had received a booster dose of that vaccine at the time.

Most side effects were mild to moderate and short-lived. The frequency of local and systemic reactions was similar after the second dose and booster.

However, myocarditis was reported less frequently after the booster dose than the second dose.

All confirmed cases of myocarditis occurred in adolescent boys. Around 84 percent of these cases were hospitalized; no deaths were reported.

The study was published on March 4, 2022, in the CDC’s Morbidity and Mortality Weekly Report (MMWR).

An earlier study by CDC researchers also found that heart inflammation was highest after the second dose and among males 12 to 24 years old.

Researchers examined data from the Vaccine Adverse Event Reporting System (VAERS) for the period December 2020 through August 2021.

Among 192 million Americans who received an mRNA COVID-19 vaccine during that time, around 1900 cases of myocarditis were reported to VAERS. Around 1600 of those met the strict definition of myocarditis.

The results were published January 25 in JAMA.

In a study published Oct. 4, 2021, in JAMA Internal Medicine, researchers examined the electronic health records of over 2.3 million Kaiser Permanente Southern California members ages 18 and older who received at least one dose of the Pfizer-BioNTech or Moderna-NIAID vaccine between December 2020 and July 2021.

They identified 13 cases of myocarditis after the second dose — a rate of 5.8 cases per million second doses given.

Two cases occurred after the first dose, making for a much lower rate of 0.8 cases per million first doses.

“[This study] supports that this is a real phenomenon,” said Dr. Ira Taub, a pediatric cardiologist at Akron Children’s Hospital, who was not involved in the research. “The risk, though, is very, very small.”

All 15 cases of myocarditis identified by researchers occurred in men between 20 and 32 years old, with an average age of 25. None of the men had a history of heart problems.

About half had received the Pfizer-BioNTech vaccine, with the other half receiving the Moderna-NIAID vaccine.

All but one of the men reported chest pain between 1 and 5 days after vaccination.

The men were all hospitalized, but their symptoms improved with conservative treatment. None of them were admitted to the ICU or readmitted after being released from the hospital.

This study included a diverse population: 37.8 percent were Hispanic, 31.2 percent white, 14.3 percent Asian, and 6.7 percent Black. This helps generalize the results to the country as a whole.

Researchers also compared the risk of myocarditis after vaccination to the risk of this condition in people who were unvaccinated.

Myocarditis can also be caused by viral and bacterial infections, including COVID-19.

The risk of heart inflammation after the second dose was 2.7 times higher than the risk in a similar group of unvaccinated people.

Dr. Brian Kolski, director of the Structural Heart Disease Program at Providence St. Joseph Hospital in Orange County, California, said that there are limitations to the study. For example, it’s an observational study rather than a randomized trial, and there was a short follow-up time.

But he said that the results fit with other data on myocarditis after COVID-19 vaccination.

“There’s nothing in this study that suggests we’re missing an epidemic of young males who are having problems with the vaccine,” he said.

Another limitation of the study is that it only included people 18 years and older. Myocarditis can also occur in younger people after vaccination.

Still, “if they’d included a pediatric population, the study would have shown substantially the same thing,” said Taub. “We are not seeing an avalanche of pediatric myocarditis cases as a result of vaccination.”

As with adults, children, and adolescents who develop myocarditis after vaccination tend to recover quickly.

“Out of the children we’ve seen [at my institution], a couple needed to be hospitalized,” he said. “But everyone was discharged within a few days, no one was re-hospitalized, and all of their [heart] function was normal.”

Experts say the benefits of the vaccines still outweigh the rare risk of myocarditis, especially when compared to the risks of coronavirus infection.

In an Israeli study published in Sept. 2021, researchers examined the electronic health records of over 2 million people.

They found an additional 2.7 cases of myocarditis for every 100,000 people vaccinated with an mRNA COVID-19 vaccine.

But the risk of myocarditis after COVID-19 was higher, resulting in an extra 11 cases for every 100,000 people who had the infection.

This study didn’t take into account younger males specifically.

But the CDC estimated in June 2021 that there would be 56 to 69 cases of myocarditis for every million 12-to-17-year-old males who received the second dose.

However, vaccination among this age group would prevent an estimated 5,700 cases of COVID-19, 215 hospitalizations, and two deaths.

Taub says that people should keep in mind that children and adolescents who develop myocarditis after coronavirus infection can get very sick.

However, “if they are treated, they bounce back pretty quickly,” he said. “We’re seeing kids in the clinic 6 months, 8 months, sometimes 10 months [after treatment], and for most of those kids, their [heart] function also eventually normalizes.”

Kolski said even with the small risk of myocarditis after vaccination, he’d still recommend that people get vaccinated against COVID-19.

However, given the greater risk of myocarditis in younger males after the second dose, two doses of the mRNA vaccine may not be appropriate for everyone.

“Maybe there’s a group of young, healthy males that only need one dose,” he said. “I think there’s some more nuanced conversations that have to occur.”

Both the United Kingdom and Hong Kong have opted to give certain adolescents a single dose of the mRNA vaccine. This provides them with some protection against COVID-19 but with a lower risk of myocarditis.