New research has found that patients who developed inflammation of the heart, or myocarditis, after receiving an mRNA vaccine for COVID-19 had fewer complications in the 18 months after hospitalization than those who developed it from contracting COVID-19 or some other cause.

The population study, conducted in France and published online Monday in JAMA^[1], found that hospital readmission rates, diagnoses of other heart-related conditions, or instances of death were significantly lower in the group that received the vaccine.

The long-term effects of COVID-19 and COVID-19 vaccines are a concern for the U.S. military, which saw nearly 500,000 cases of the coronavirus^[2] in the first two years of the pandemic and required all members to get the vaccine^[3].

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The study findings were the same regardless of age, according to the research, although the authors noted that post-vaccine myocarditis patients, who were mainly healthy young men, "may require medical management up to several months after hospital discharge."

"Although patients with myocarditis after COVID-19 mRNA vaccination appear to have a good prognosis near hospital discharge, their longer-term prognosis and management remain unknown," wrote the researchers, led by Laura Semenzato, a statistician with EPI-PHARE Scientific Interest Group.

The Defense Department vaccinated more than 2 million service members from December 2020, when the U.S. Food and Drug Administration issued an emergency use authorization for the Pfizer and Moderna COVID-19 vaccines, through December 2022, according to the Pentagon.

The vaccines used a novel mRNA technology to stimulate the body's immune response to the illness, using messenger RNA to instruct cells to make pieces of the virus' spike protein to trigger an immune response.

As early as February 2021, the U.S. military began seeing patients who developed myocarditis after receiving their COVID-19 vaccine series.

Military.com began tracking the cases^[5] shortly after a civilian member of the Pentagon press corps and a Military.com reporter who serves in the National Guard^[6] developed the condition, although the Pentagon did not confirm it had additional cases until April.

In a report to Congress last September on troop health following the Defense Department's COVID-19 vaccine mandate, which went into effect in August 2021, a Pentagon official said 25 service members developed myocarditis in early 2021 but did not give a total number of cases for the force over the years.

The report said the rate for service members developing myocarditis was 57 cases per 100,000 "person years," which measures the number of people across the observation time, versus 98 cases per 100,000 person years among those who contracted COVID-19 in 2021.

The Department of Veterans Affairs^[7], which could be asked to provide disability compensation for veterans who have long-term health effects from COVID-19 contracted on active duty or from the vaccine, has 11 ongoing studies on long-term coronavirus.

"The VA research program remains tightly focused on understanding the long-term impacts

of COVID-19," wrote VA Under Secretary for Health Dr. Shereef Elnahal in an article in Federal Practitioner last November. "At the same time, the VA is committed to using lessons learned during the crisis in addressing high priorities in veterans' health care."

Dr. Harlan Krumholz, a cardiologist at Yale School of Medicine^[8] who was not involved in the French study, said the latest research provides reassurance to patients and doctors about the possible long-term effects on the heart and body of post-vaccine myocarditis.

But, he noted, while the study focuses on the outcomes at 18 months after hospitalization -- what Krumholz described as "meaningful follow-up," it does not provide insight into longer-term outcomes.

"In general, myocarditis at a young age can potentially lead to chronic heart issues like arrhythmias or heart failure in some patients, though many recover fully. Ongoing monitoring is important," Krumholz wrote in an email Tuesday to Military.com.

For the study, researchers examined 4,635 patients ages 12 to 49 who were hospitalized for myocarditis in France from Dec. 27, 2020, to June 30, 2022.

Of those, 12% developed post-vaccine myocarditis within seven days of getting the immunization while 6% developed post COVID-19 myocarditis and 82% had a conventional form of the condition.

While the number of patients who developed myocarditis after contracting the illness was smaller than the vaccine group, they were hospitalized and had rates of complications or death similar to those who developed regular myocarditis.

The study noted, however, that one patient with post-vaccine myocarditis required extensive medical interventions and died after leaving the hospital, with myocarditis likely the cause of death.

"While outcomes were generally favorable, some patients required ongoing medical management for several months after discharge. Also, 3% of those who had post-vaccine myocarditis were rehospitalized ... over the subsequent 18 months," noted Krumholz after reviewing the study.

Myocarditis can result from a viral infection or an overactive immune response to an illness. The reason why some people, especially young men, develop myocarditis after getting a COVID-19 mRNA vaccine is not well understood. Krumholz said it likely involves an "exaggerated immune response," occurring in roughly 1 to 10 of every 100,000 vaccinated individuals.

The JAMA study has its limitations, as researchers were able to look only at hospitalizations for myocarditis and not all potential cases during the time frame. They also did not include details on the severity of cases and based their research on medical diagnoses, relying on the accuracy of medical providers.

The researchers noted that the American Heart Association and the American College of Cardiology guidelines advise patients with myocarditis to refrain from competitive sports for 3 to 6 months and to have their health condition assessed prior to the resumption of sports.

They also said that, while several studies have reported "reassuring results" for the prognosis of post-vaccine myocarditis, patients with the low likelihood of poor outcomes, residual symptoms and cardiac abnormalities have been detected up to a year after illness.

The study, the researchers said, should be "taken into account for ongoing and future mRNA vaccine recommendations."

"Overall, this study provides important data on medium-term outcomes, but continued research on longer-term prognosis is still needed," Krumholz added.

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