Americans Brooks and Chase Koepka, Webb Simpson and Cameron Champ - plus Northern Ireland's Graeme McDowell - withdrew from the ongoing Travelers Championship.

They made the decision after Champ tested positive for coronavirus, which forced him to drop out.

Nick Watney had already returned a positive test last week, while the caddies for Brooks Koepka and McDowell also tested positive. 

The PGA Tour is subsequently making adjustments to its testing protocols and other anti-virus measures. 

Those who travel via the Tour-procured charter flight will be subject to arrival testing procedures, in addition to the pre-charter test.

Player instructors have been added to the on-site testing protocol, while the Tour will try to eliminate the use of off-site gyms. 

The Players Advisory Council has also suggested that each tournament provide nightly dinner service in order to give players a safer option than going out to eat. 

"I think when you get into the environment of the tournament with no spectators here, with very few people here, with people that are around you having tested negative, I think over the first couple weeks, we've seen some instances where, let's say we've gotten a little bit lax or away from protocol," said PGA Tour commissioner Jay Monahan.

"Full disclosure: I've done it myself, and I think that's the kind of tightening that we need to do in order to make sure we continue to be in a good position to move forward.

"Safely moving forward, of course, may mean further adjustments when necessary. 

"For now, the Tour is confident in the program and protocols currently in place.

"We've learned a lot.

"We are continuing to refine… it's all about trying to live in this world and be able to sustain your return in a world of COVID-19.

"I sincerely feel like we are on the right path in that regard."

The PGA Tour resumed last month with the Charles Schwab Challenge in Texas. 

Play had been suspended since March due to the pandemic.