SAN DIEGO (KGTV) -- A new study on members of the U.S. military who received the COVID-19 vaccines adds more evidence to a link between the shots and rare cases of heart inflammation, but the researchers said their findings should not diminish overall confidence in the shots.
From January through the end of April, the military administered 2.8 million doses of the vaccines from Pfizer and Moderna.
The study, published in JAMA Cardiology, describes how 23 service members later developed chest pain and sought medical attention. Testing showed it was myocarditis.
Myocarditis is inflammation of the heart, specifically within the middle layer of heart muscle.
Last week, an advisory committee to the Centers for Disease Control and Prevention said there was a "likely association" between about 1,200 reports of heart inflammation and the mRNA COVID-19 vaccines made by Pfizer and Moderna.
The panel concluded the vaccines’ benefits still greatly outweigh the potential risks, but the Food and Drug Administration added a new warning to the literature that accompanies the shots highlighting that rare and typically mild cases of heart inflammation had been detected.
Most of the cases have been in boys and young men.
In the military study, all 23 cases were men, all happened within four days after vaccination and most happened after the second dose. Most of the men were service members in their 20s.
To examine whether the vaccines were the likely cause, researchers compared the number of cases to the background rate. The background rate is the number of cases of myocarditis that would be expected from other causes.
Myocarditis can be triggered by a number of things, including infection. “There are like 15 or 20 common offenders,” said Dr. Christian Ramers of Family Health Centers of San Diego. “Things like chickenpox can do it. Things like enteroviruses that cause infections in kids can do it.”
In the days following the second shot, the researchers said they would expect to find between 0 and 8 cases of myocarditis from other causes. Instead, they found 19, suggesting a possible link to the vaccines.
“The majority of people will not only get better, but you'll get better very quickly,” said Dr. Peter Chin-Hong, an infectious disease specialist at UC San Francisco.
Myocarditis can involve sharp chest pain or shortness of breath at first, but it usually resolves fast, Dr. Chin-Hong said. Some patients heal without treatment. Others may require simple anti-inflammatory medicine. In rare cases, the inflammation can cause permanent damage to the heart.
In the military study, 16 of the 23 patients recovered within one week. Seven patients continued to have chest discomfort at the time the report was published.
Doctors aren’t sure why the condition is more common in boys and young men, but the cases in healthy, fit military members are renewing questions about the role of exercise.
Vigorous exercise can worsen myocarditis. “If you increase the blood flow needed during exercise, the heart really can't keep up,” said Dr. Chin-Hong.
In rare cases, that can lead to heart failure.
People who develop myocarditis are told to avoid vigorous exercise and sports for three to six months so the heart has time to heal, but some doctors are beginning to wonder if exercise just after vaccination could have an impact.
“I think that raises a very important question,” said Dr. Abisola Olulade of Sharp-Rees Stealy Family Medicine. “Do we recommend that people in this age range not exercise after they get the vaccine? I think it’s an important question because there is data that if someone is experiencing myocarditis they should not exercise because it can make it worse.”
So far, there is no such guidance that people avoid exercise in the days after the shot, Dr. Olulade noted.
Scientists have yet to identify a mechanism for how the vaccines might trigger the inflammation.
People infected with COVID-19 have developed heart complications. Nearly one percent of college and pro athletes infected with COVID showed signs of myocarditis in scans, the military researchers noted.
That means infection poses a much higher risk of heart problems than vaccination, Dr. Chin-Hong said.
The latest estimates suggest about 12 cases of myocarditis after every million doses of vaccine, “which is a similar risk to drowning while swimming,” he noted.
The authors of the military study said their findings should not discourage people from seeking the life-saving protection the vaccines offer.
“Concerns about rare adverse events following immunization should not diminish overall confidence in the value of vaccination,” the authors wrote.