According to a population-based study from France, people who developed myocarditis after vaccination did not have the typical medium-term complications associated with conventional myocarditis.

An 18-month follow-up of people hospitalized for myocarditis during the COVID-19 pandemic found that combined clinical outcomes were more favorable when people developed myocarditis shortly after COVID-19 mRNA vaccination rather than conventional myocarditis (weighted HR 0.55, 95% CI 0.36–0.86).

Meanwhile, the risk of a poor composite outcome – taking into account rehospitalization for myopericarditis, other cardiovascular events and death from any cause – was comparable between the groups with myocarditis after COVID-19 and those with conventional myocarditis (weighted HR 1.04, 95% CI 0.70-1.52), said Laura Semenzato, MSc, statistician at Saint-Denis Cedex France in Paris, and colleagues.

These results apply equally to young people and older myocarditis patients. “However, affected patients, mainly healthy young men, may require medical care for up to several months after discharge from hospital,” Semenzato’s group explained in JAMA.

The authors added that their findings, based on the French national health data system, are consistent with previous data linking post-vaccination myocarditis with a lower short-term risk of heart failure compared with other types of myocarditis.

“These data confirm the largely favorable prognosis of patients with myocarditis following vaccination. Extended follow-up provides further reassuring evidence that while this is a real condition, very few patients suffer long-term clinical consequences,” commented Dr. James de Lemos, a cardiologist at UT Southwestern Medical Center in Dallas.

The results support the notion that “vaccine-induced myocarditis is fundamentally different and poses a much lower risk than COVID-induced myocarditis and non-COVID, non-vaccine-induced myocarditis,” he said. MedPage Today.

Although the preponderance of evidence suggests a causal link between the first mRNA vaccines against COVID-19 and myocarditis – which is more common in young men and adolescent boys – post-vaccination myocarditis is rare and relatively mild. Reports suggest that recovery typically takes a few days in hospital, and fears of excess sudden cardiac deaths among those affected have not yet been realized.

“Given the effectiveness of the vaccines, these facts clearly suggest that (coronavirus) vaccines remain a safe and critical part of combating this now endemic virus,” said cardiologist James Januzzi Jr., MD, of Massachusetts General Hospital and Harvard Medical School, both in Boston.

In fact, major health authorities continue to endorse COVID vaccines for prevention in children and adults.

Last week, the FDA approved and granted emergency use authorization to updated vaccines from Moderna (Spikevax) and Pfizer-BioNTech (Comirnaty) for people 6 months and older. These new vaccines target the KP.2 strain of SARS-CoV-2 to better combat the variants currently circulating.

CDC Director Mandy Cohen, MD, MPH, immediately advocated giving these vaccines to everyone 6 months of age and older.

“I routinely tell my patients to get vaccinated when appropriate. Since the emergence of a new variant, COVID rates have been rising quite sharply. The good news is that a new version of the vaccine is now available that is tailored to the variant currently circulating,” Januzzi wrote in an email.

For their French cohort study, Semenzato’s group used a database that included all people aged 12 to 49 who were hospitalized for myocarditis between December 27, 2020, and June 30, 2022.

A total of 4,635 individuals were included: 12% with post-vaccination myocarditis (within 7 days of COVID-19 mRNA vaccination), 6% with post-COVID-19 myocarditis (within 30 days of SARS-CoV-2 infection), and 82% with conventional myocarditis (remaining cases).

Patients with post-vaccination myocarditis were younger and more often men than in the other groups. Two-thirds of cases of post-vaccination myocarditis occurred after a second dose of vaccine.

Trends in medical treatment of patients after hospitalization were similar in all three myocarditis groups.

The authors found that one patient suffered from post-vaccination myocarditis and required extracorporeal membrane oxygenation. After discharge from the hospital, the patient died. Myocarditis was considered the most likely cause of death.

Semenzato and colleagues said the 18-month follow-up was completed in 99% of the population.

However, despite the large sample size, they cautioned that they were unable to capture cases of myocarditis that did not require hospitalization and that results may vary depending on the definition of post-vaccination myocarditis.

An important question that remains unanswered in this field is why some people develop myocarditis after COVID-19 mRNA vaccination and others do not.

“The mechanisms leading to myocardial injury following administration of a COVID-19 mRNA vaccine are not yet well understood. There are several hypotheses such as altered gene expression, direct immune activation by mRNA, molecular mimicry, immune dysregulation or aberrant cytokine expression,” wrote Semenzato and colleagues.