Israeli research: Myocarditis after Pfizer booster remains rare and mild

Hadassah-University Medical Center/Health Ministry research puts end to claims about "serious and frequent" heart muscle inflammation from COVID-19 shots.

 People pose with syringe with needle in front of displayed Pfizer logo in this illustration taken. (photo credit: REUTERS)
People pose with syringe with needle in front of displayed Pfizer logo in this illustration taken.
(photo credit: REUTERS)

Opponents of the COVID-19 mRNA vaccines made a big fuss almost a year ago about alleged serious and widespread cases of myocarditis resulting from the vaccination. Now, Israeli research has disproved once and for all that such a side effect was common and severe.

Myocarditis is inflammation of the heart muscle (myocardium). The inflammation can reduce the heart’s ability to pump blood, and the condition can cause shortness of breath, chest pain and rapid or irregular heart rhythms (arrhythmias).

Infection with a virus is one cause of myocarditis, but sometimes a drug reaction or general inflammatory condition triggers it. Severe myocarditis weakens the heart so that the rest of the body does not get enough blood. Clots can form in the heart, leading to a stroke or heart attack.

The new study found that the risk of developing myocarditis among males 16 to 19 years old was one in 15,000 after a third dose of the Pfizer-BioNTech vaccine, and the cases were rare and mild. The study was published in the American Heart Association’s flagship journal Circulation.

Research detailing post-vaccination myocarditis in Israel after the first and second doses of the Pfizer-BioNTech COVID-19 vaccine was recently published by the Health Ministry in Jerusalem.

The incidence rate of myocarditis was low and primarily in young males after a second COVID-19 vaccination, suggesting a potential relationship between the vaccine and myocarditis.

The results raised concerns about the potential for increased myocarditis after a booster dose, so this new analysis was focused on the risk of myocarditis after a booster dose.

“It is important to understand the connections between this rare heart condition and COVID-19 vaccines, so we can monitor the prevalence of myocarditis and pay extra attention to those who are most at risk,” said lead study author Prof. Dror Mevorach, an internal medicine specialist and head of immunology-rheumatology at Hadassah-University Medical Center in Jerusalem’s Ein Kerem. He is also chairman of the ministry’s health committee for identifying myocarditis as an adverse effect of mRNA vaccines.

Several previous studies and reports from public health agencies around the world, including the US Centers for Disease Control and Prevention, have highlighted a possible connection and potentially increased risk of myocarditis after receiving an mRNA COVID-19 vaccine, generating considerable scientific, policy and public interest.

An episode of myocarditis may resolve on its own or with treatment, or it may result in lasting damage to the heart. In the general population not during a global pandemic, it is estimated that approximately 10 to 20 people per 100,000 are diagnosed with myocarditis each year, according to the American Heart Association’s 2021 scientific statement on myocarditis.

From July 31, 2021, to November 5, 2021, nearly four million Israeli adults received a booster dose of the Pfizer vaccine, about half (48.7%) of whom were males. Health data for all reported cases of myocarditis after receiving the shot were evaluated. A cardiologist and a rheumatologist reviewed and classified the data.

The researchers found that after a 30-day follow-up, only 91 cases of myocarditis were reported, including 35 cases that occurred within the first 30 days of receiving a COVID-19 booster (a third dose) of the Pfizer-BioNTech vaccine.

Twenty-eight cases of myocarditis were probable or confirmed, and 18 occurred within the first seven days after receiving the booster dose of the Pfizer-BioNTech vaccine. All 28 cases of myocarditis were clinically defined as mild, and individuals recovered within an average of three and a half days in the hospital.

Males at a higher risk

Among all age groups, the risk rates of developing myocarditis were nearly nine times higher in males than in females.

Males 16 to 19 years old were at the highest risk, with six in 100,000 individuals developing myocarditis, followed by males ages 20-24 (5.21 cases per 100,000), 30-39 (1.81 cases per 100,000) and 25-29 (0.79 cases per 100,000). The risk differences declined significantly between the second and third vaccine doses across both genders and all age groups.

The authors believe there are two potential explanations for the changes. The first is that individuals who developed myocarditis after the second COVID-19 vaccine dose did not receive a third shot, which was a medical precaution in Israel, they said.

The second potential explanation is the interval of time between doses. First and second doses are given about three weeks apart, but the time between a second dose and a booster was about 20 to 24 weeks, Mevorach said.

Researchers believe further study is required to better explain what may predispose young males to develop myocarditis after a COVID-19 vaccine and the pathophysiological mechanisms involved.