Three players who continue to test positive for COVID-19 teed off together in Thursday's opening round of the US PGA Workday Charity Open under "clarified" rules for symptomatic participation.

South Africa's Dylan Frittelli and Americans Denny McCarthy and Nick Watney began with pars at the 10th hole at Muirfield Village in their morning start after having limited access to facilities at the Ohio, layout.

The PGA Tour protocol guidelines involve players and caddies who meet the US Centers for Disease Control (CDC) "Return to Work" guidelines after consultation with infectious disease experts and PGA medical advisor Dr. Tom Hospel.

Those allow condition returns to competition for players and caddies who continue to test positive for coronavirus if at least 10 days have passed since the symptoms first appeared and 72 hours have passed since fever has disappeared and issues such as cough and shortness of breath have improved.

"In the beginning stages of the illness, that virus is assumed to be active virus that can cause infection, can be contagious," Hospel said. "As time passes and as symptoms resolve (the) theory is that this virus, this particle that has being detected... is no longer active or contagious. What we've learned along the way is that in some instances, individuals can continue to test positive for weeks if not months beyond when their illness started, and the thought is that those individuals are no longer contagious."

A 10-day isolation was required for those testing positive but under July 1 revisions, an infected player or caddie can return if there are no symptoms displayed once two negative tests are taken at least 24 hours apart.

The CDC and experts say tests used by the tour show the possibility of detecting virus even after the infectious aspect of it is gone and a person is no longer contagious. The PGA decided players and caddies who meet that criteria will be grouped together or play alone and have no access to indoor facilities "out of an abundance of caution."

"They have complied with the guidance from the CDC, they have met the medical requirements for isolation, and with respect to the opinions of our medical advisors, including infectious disease experts and the CDC, they're clear to play," PGA Tour vice president for administration Andy Levinson said.