Only five of the 789 U.S. professional athletes who tested positive for the coronavirus in 2020 developed serious heart disease, and none had significant issues after being cleared to return to play, according to an unprecedented, collaborative study among MLB, MLS, the NBA, the NFL, the NHL and the WNBA that was published Thursday in a leading medical journal.

The study, published in the journal JAMA Cardiology, was written by the top medical officials of those leagues and grew out of the weekly teleconferences those officials began holding at the onset of the pandemic in March 2020.

“We felt it was really important to share our best practices as we were all struggling with the same things,” said Gary Green, medical director for MLB and one of the study’s authors. “Once we realized we had these numbers [of cases], we started talking to the various cardiologists who work with our different leagues and players’ associations to come up with a collaborative effort.”

The study, conducted using data from athletes who tested positive between May and October of last year, found athletes developed serious heart disease at a rate far lower than the general population. Though only 0.6 percent (five of 789) of the athletes were found to have myocarditis or pericarditis, those conditions were found in more than 7 percent of all coronavirus-positive patients, according to a study published last year in Circulation, a journal of the American Heart Association.

“We did this because, at the beginning of [the pandemic], it was clear that people who were hospitalized with covid did have a significant [incidence] of heart disease,” Green said in a telephone interview. “And we wanted to make sure athletes who got covid were returning to their sport and were doing it in a safe manner. … We wanted to see: Is it safe for [athletes] to return, and what’s the risk? What we found is that the risk is very, very low.”

Of the 789 athletes who tested positive for the coronavirus, 460 (or 58.3 percent) were considered symptomatic, with the rest asymptomatic. Only 30 athletes, 23 of whom came from the symptomatic group, were referred for additional cardiac testing, and only five of those were diagnosed with inflammatory heart disease: three with myocarditis, or inflammation of the heart muscle, and two with pericarditis, or inflammation of the tissue surrounding the heart.

Those five were held out of playing their sport for varying lengths of time, and some have since returned without incident.

“As of late December 2020, no clinical cardiac events have occurred in any of the athletes who have undergone cardiac screening and resumed full professional sporting activity,” the study concludes.

The study did not name the professional athletes found to have suffered heart disease after being diagnosed with the coronavirus because of privacy laws. But at least two have been publicly identified in news reports: Boston Red Sox pitcher Eduardo Rodriguez, who missed the entire 2020 MLB season after being diagnosed with myocarditis but is participating in spring training ahead of the 2021 season, and Buffalo Bills tight end Tommy Sweeney, who was already out with a broken foot when he was diagnosed with myocarditis in November, ending his 2020 season.

The study makes note of its inherent limitations, including its retroactive nature and lack of uniform methodology for screening and diagnosing subjects; the fact it was written before the arrival of newer, potentially more infectious and potent coronavirus variants; and the youth and general fitness of its subject group, relative to the general public. In addition, 98.5 percent of the study’s subjects were men.

“We wanted to be very careful [to say] that we don’t mean we can extrapolate this to everyone else,” Green said.

Still, the study’s authors achieved their primary mission, according to Green.

“One of the goals of all the leagues has been to contribute to the literature and the understanding of covid,” he said. This study, he added, “does that.”