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  • After reviewing data on cases of myocarditis and pericarditis in people vaccinated with an mRNA COVID-19 vaccine, scientists say the benefits still outweigh the risks.
  • Myocarditis is inflammation of the heart muscle, while pericarditis is inflammation of the two-layered sac surrounding the heart.
  • These conditions can also occur naturally after other viral infections, but cases in people vaccinated with mRNA COVID-19 vaccines were slightly higher than average.

The Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC) are investigating reports that the Moderna-NIAID vaccine is associated with a higher risk of myocarditis for younger adults than previously believed, according to an August 20 story by The Washington Post.

However, even with a higher rate, this condition is still uncommon.

The data that the agencies are reviewing were provided by the Canadian government.

They suggest the rates of myocarditis after the Moderna-NIAID vaccine may be two-and-a-half times higher than after the Pfizer-BioNTech vaccine, the Post reported.

The risk was greater for males under the age of 30.

The agencies are also reviewing data from the United States. One of the anonymous sources who spoke to the Post said it’s too early to reach a conclusion about the new data.

U.S. scientists said in June that there’s a “likely association” between mRNA COVID-19 vaccines and a higher risk of rare heart inflammation in adolescents and young adults.

Still, the benefits of the vaccines outweigh the risks, said members of the CDC’s Advisory Committee on Immunization Practices (ACIP). They recommend vaccination for all Americans 12 years and older.

The CDC’s ACIP reviewed data on over 1,200 cases of myocarditis or pericarditis in Americans following vaccination with the Pfizer-BioNTech or Moderna-NIAID COVID-19 vaccine.

Myocarditis is inflammation of the heart muscle, whereas pericarditis is inflammation of the two-layered sac surrounding the heart.

Reported symptoms included chest pain and difficulty breathing, which typically began within a week after vaccination. People also showed abnormal electrocardiogram and blood test results.

Dr. Pranav Patel, the chief of cardiology and an interventional cardiologist with UCI Health, said in an earlier interview that myocarditis and pericarditis can occur naturally due to viral and other infections.

“But the rate seen after [the mRNA] vaccines is slightly higher than would be expected for younger individuals,” he said. “This is what caused the CDC to look more closely into this.”

Most cases reported to the CDC occurred in people under age 30 — particularly men — and after the second dose. Researchers do not know why.

However, this side effect of vaccination is uncommon.

In those 12 to 39 years old, heart inflammation occurred at a rate of 12.6 cases per million second doses given. The rate after the first dose was 4.4 cases per million doses administered.

Both rates were measured within 21 days after vaccination.

The rates were higher among people who received the Moderna-NIAID vaccine compared with the Pfizer-BioNTech vaccine.

Health officials in the U.S. Defense Department and Israel have reported similar cases of myocarditis or pericarditis among younger men following vaccination with an mRNA vaccine.

Patel said that, because the chance of heart inflammation is extremely low, the CDC is still recommending that all people 12 years old and older receive a COVID-19 vaccine.

However, if a person develops myocarditis or pericarditis after the first dose of an mRNA vaccine, the agency recommends that their second dose be delayed.

After a person’s heart is fully healed, the CDC said a second dose of an mRNA vaccine could be considered under certain circumstances.

In June, the FDA added a warning to the patient and provider fact sheets for the mRNA vaccines, explaining that cases of heart inflammation are a rare, but potential, side effect.

At least 1,226 cases of myocarditis or pericarditis have been reported as of June 21, according to data presented at a CDC meeting.

Of the reported cases, 484 occurred in people between 12 and 29 years old. Health officials are still investigating to confirm some of these.

Among the verified cases in younger people, 309 were hospitalized. As of June 11, only 9 were still hospitalized, with 2 in intensive care, the agency said.

About 81 percent of those who have been discharged from the hospital had recovered from their symptoms at the time of the report.

No deaths have been associated with these vaccine-related conditions.

Compared with myocarditis not related to a vaccine, the cases that occurred after mRNA vaccination were milder, with a shorter duration and minimal treatment needed, Dr. Matthew Oster of the CDC told the committee.

Similarly, Dr. Michael Chan, an interventional cardiologist with Providence St. Joseph Hospital in Orange, California, said most cases of myocarditis after the COVID-19 vaccination that he’s seen have been mild.

“These children have not been severely involved with the myocarditis,” he said, “like some of the patients that I’ve seen with seasonal viral myocarditis.”

These people didn’t require medications to support their blood pressure or to treat an abnormal heart rhythm. Instead, they were given a non-steroidal anti-inflammatory medicine, such as ibuprofen.

After that, “their chest pain went away after a day or two, the heart muscle tests trended down toward normal, and they were discharged,” Chan said.

Officials at the CDC meeting said warnings about the potential risk of myocarditis and pericarditis should be added to the fact sheets provided to healthcare professionals and vaccine recipients

Chan said the benefits of the vaccine clearly outweigh the risks, even with the low risk of myocarditis after vaccination.

While children are less likely than adults to become severely ill from COVID-19, some children are hospitalized or die from the illness.

They can also develop a rare but serious medical condition called multisystem inflammatory syndrome in children (MIS-C).

COVID-19 can also cause heart problems in young people. A 2021 study in JAMA Cardiology found that 2.3 percent of collegiate athletes who recovered from COVID-19 had symptoms of myocarditis.

“COVID-19 infection is much more dangerous than the possibility of getting myocarditis or pericarditis from the vaccination,” Patel said.

Following the CDC meeting, top administration health officials joined with other health groups to endorse the use of mRNA COVID-19 vaccines in adolescents and young adults.

Currently, COVID-19 vaccines are approved in the United States for people 12 years old and older.

“The facts are clear: [heart inflammation] is an extremely rare side effect, and only an exceedingly small number of people will experience it after vaccination,” CDC Director Rochelle Walensky and others wrote in the statement.

“For the young people who do, most cases are mild, and individuals recover, often on their own or with minimal treatment. In addition, we know that myocarditis and pericarditis are much more common if you get COVID-19, and the risks to the heart from COVID-19 infection can be more severe.”

Chan said that, because the coronavirus is unlikely to disappear any time soon, vaccination remains important, especially with the rapid spread of the Delta variant.

“I would strongly encourage folks to get vaccinated as soon as possible,” he said. “Vaccination is critical in preventing further mutations of the virus into potentially even deadlier COVID-19 variants in the future.”