Heart problems following vaccination in the U.S. are uncommon and short-lived, researchers reported.

For every one million Americans immunized with a coronavirus vaccine, about 60 develop temporary heart problems, according to a study published on Wednesday in the journal JAMA Network.

The complications were all short-lived, the researchers found. And these heart problems are far more common among patients who develop Covid-19, outside experts noted.

Analyzing the medical records of just over 2 million people who had received at least one dose of a coronavirus vaccine through May 2021, the new study found 20 cases of myocarditis, or inflammation of the heart muscle, and 37 cases of pericarditis, inflammation of the membrane surrounding the heart.

Patients who were hospitalized were discharged after only a few days, and none died.

The incidence of myocarditis in the study, at 10 cases per million vaccinated, is higher than the Centers for Disease Control and Prevention’s estimate of 4.8 cases per million, suggesting that there may be more cases than are reported to the federal database for tracking so-called adverse events following vaccinations.

“We see that these adverse events are leading to very short and unremarkable hospital stays,” said Dr. Jeremy Faust, an emergency medicine physician at Brigham and Women’s Hospital in Boston who was not involved in the study. “The same can’t be said of hospitalizations for Covid-19 in this or any age group so far.”

“When people are hospitalized for Covid, the consequences are far more severe,” added Dr. Faust, who has compared rates of myocarditis following vaccination to those among Covid-19 patients.

The researchers, with the Providence health care system, evaluated medical records from 40 hospitals in Washington, Oregon, Montana and Los Angeles County, Calif.

They found that myocarditis developed a median of 3.5 days after vaccination, mostly after the second dose, and in people with a median age of 36 years. Three-quarters of the 20 cases were in men.

The 19 patients who were admitted to the hospital were discharged after a median of two days. About three weeks after vaccination, 13 patients had recovered from their symptoms and the remaining seven were improving.

Pericarditis affected older patients, at a median age of 59 years, and later, about 20 days after vaccination, the researchers found. Pericarditis, too, was more common in men. Of the 37 cases identified, 13 were admitted to the hospital; the median stay was one day.

A separate study, posted online last week, suggested that the incidence of myocarditis in boys ages 12 to 17 who had Covid-19 was 876 per million; in girls of the same age group with Covid-19, the incidence was 213 cases per million.

The study has not yet been peer reviewed or published in a scientific journal.