There are few cases of inflammatory heart disease among professional athletes who suffer mild cases of COVID-19, according to a study authored by medical experts from Major League Baseball, the National Basketball Association and other major North American sports leagues.

Medical personnel from MLB and the NBA, along with the National Football League, National Hockey League, Major League Soccer and the Women's National Basketball Association pooled data from athletes from May through October 2020, as professional sports in North America slowly returned to action after coronavirus-linked lockdowns.

The retrospective study, the first of its kind among the six leagues and published by JAMA Cardiology on Thursday, showed that five of 789 athletes who tested positive for COVID-19 during that time were found to have inflammatory heart disease after mandatory "return-to-play" cardiac testing.

"It does show that in this population of athletes it is safe to return and that inflammatory heart disease is relatively uncommon," said Dr. Gary Green, medical director for MLB, who confirmed that all were able to return to play.

None of the people in the study were hospitalized due to COVID-19 infection and none would have been classified as "seriously ill," Green told Reuters.

More than half a million people in the United States have died from the novel coronavirus, as government officials scramble to roll out vaccines and stem the spread of the highly contagious virus.

"The study was sort of designed to answer the uncertainty that there was surrounding this issue about whether cardiac inflammation was present in people who had mild or asymptomatic forms of COVID-19 illness," said Dr. David Engel, the consulting cardiologist for the NBA and a co-author of the study.

Numerous professional athletes have opted out of playing since the COVID-19 pandemic first took hold in North America, amid concerns over the long-term impact of the virus.

Boston Red Sox pitcher Eduardo Rodriguez missed the 2020 season after contracting the novel coronavirus and subsequently being diagnosed with myocarditis. He since said he plans to play in 2021.

"Being athletic doesn't necessarily protect them anymore than if they were sort of an age-matched non-athlete," said Engel. "The cardiac inflammation that you can see with COVID-19 or really with any virus poses a unique risk to athletes because myocarditis, if it's present, it can cause a lethal or a dangerous arrhythmia when the heart is stimulated."

The report offered little insight into the impact of the novel coronavirus on female athletes, with the WNBA the only North American women's league included in the study. Just 12 WNBA players tested positive for COVID-19 during the study's timeframe.