Getting at least one dose of the Pfizer vaccine increased the risk of a rare type of heart inflammation, especially in young males, according to a large new study from researchers in Israel. However, the diagnosed cases were usually mild, and most of the patients were sent home without ongoing need for treatment.
The condition, called myocarditis, an inflammation of the heart muscle, has been reported in a very small number of people who received either the Pfizer-BioNTech or the Moderna mRNA vaccines. Symptoms can include chest pain, palpitations and shortness of breath. Most patients who get care recover quickly.
The study, which was published Wednesday in the New England Journal of Medicine, looked only at people who received doses of the Pfizer vaccine. Researchers identified 54 cases of myocarditis among more than 2.5 million people ages 16 years and older who got at least one dose of the Pfizer vaccine. All but three of them occurred in males. Ages were from 21 to 35, with a median of 27.
The researchers looked back at health care records from December to May at Clalit Health Services, the largest health care organization in Israel. Patients were followed for up to 42 days after the first vaccine doses.
Overall, there were 2 cases of myocarditis for every 100,000 people who were vaccinated. The rate was highest among male patients ages 16 to 29, with nearly 11 cases of myocarditis per 100,000 vaccinated people.
For full coverage of the coronavirus pandemic
The authors noted that the rate in Israel was higher than in the U.S., where the Centers for Disease Control and Prevention gather the data, most likely because of how the data were collected. The CDC has estimated that the incidence of myocarditis after any Covid vaccination is 0.48 cases per 100,000.
The differences can be explained by different age cutoffs and because the Israeli researchers examined medical records in a large health care system. The CDC relies on self-reported data in VAERS, a vaccine adverse event monitoring system.
“In all age groups, the benefits still very much outweigh the potential risks,” said Dr. Sean O’Leary, the vice chairman of the American Academy of Pediatrics’ committee on infectious diseases, who was not involved with the research.
He said that the study was “very nicely done” and that it would add to the understanding of the true risk of myocarditis after vaccinations, particularly for younger men.
“This helps us get closer to a precise estimate of the risk, but it doesn’t necessarily mean this is the exact estimate,” he said.
Most cases after the second vaccine dose
Consistent with data from the CDC, most of the cases in Israel occurred three to five days after the doses. The study did not include the third booster dose of the Pfizer vaccine, which Israel began administering during the summer.
Chest pain was the most common reported symptom, along with fever and shortness of breath. The median hospital stay was three days, with 65 percent of patients sent home without ongoing need for treatment.
Fourteen patients had issues with the left ventricle of the heart during admission; 10 were still experiencing dysfunction when they were sent home. Later testing found normal heart function in five of those patients.
More than three-quarters of the cases were described as mild, meaning the patients had symptoms but their heart function was normal. Another 22 percent with symptoms experienced some abnormality in cardiac testing.
There was one previously unreported severe case of heart dysfunction, and the patient fully recovered with treatment. One person with a history of heart problems died; the exact cause of death was unknown.
“The study reinforces the bottom-line message, which is that most of these cases are mild,” said Dr. Walid Gellad, the director of the Center for Pharmaceutical Policy and Prescribing at the University of Pittsburgh.
Gellad, who was not involved with the research, said it was very helpful in understanding the true risk of myocarditis after a Pfizer vaccine shot.
“This is a problem primarily for teenage boys and young men,” Gellad said. “It's just a repeated pattern in every study that this is a problem after the second dose primarily for young men.”
Heart risks greater from Covid infection
Dr. Yvonne Maldonado, a pediatric infectious disease physician at Stanford University, said it is important that people know that the risk of myocarditis from coronavirus infection remains higher than from Covid vaccinations.
“If you look at the risk of myocarditis, it’s much higher for people who develop Covid-19 rather than the vaccine,” she said.
A recent Case Western Reserve University study posted online estimated that the risk of developing myocarditis from infection was six times higher than from vaccination in young males. The study has not been peer-reviewed.
Maldonado said that when she speaks to families about their concerns with vaccination, she tries to be as open as possible about the risks and benefits.
“We need to be transparent with families, but we also need to lay out, as we do with every other vaccine, what are the risks and what are the benefits,” she said. “Families still come to pediatricians for this kind of guidance.”
Gellad, a primary care provider, agreed.
“It's a tough one to talk about, because you don’t want to contribute to hesitancy,” he said. “On the other hand, you have to be honest with people, and honesty is often what people respond to.”
Follow NBC HEALTH on Twitter & Facebook.
