The NFL is reminding all league personnel to report even mild symptoms and to get tested regularly in an effort to prevent large virus outbreaks.
The league issued the updated guidance on Tuesday, in a video published on social media and in a memo sent by Commissioner Roger Goodell, according to the Associated Press.
"It is vital for all players, coaches and other personnel to understand and report symptoms immediately," Goodell's memo said. "It is also essential for our medical staffs to continue their efforts to monitor everyone in the club environment."
Both the memo and the video note that the virus can still be contracted and spread by vaccinated individuals. Recent reporting from the NFL indicated 93.5% of league personnel were inoculated.
"While the vaccine virtually eliminates the likelihood of the worst outcomes of COVID-19, it is still possible for vaccinated individuals to become infected," the league said in a Twitter post accompanying the video.
"It is crucial that everyone -- players, coaches & fans -- report any symptoms and get tested for Covid-19."
In the video, the NFL's chief medical officer, Dr. Allen Sills, appears first, and is later followed by several head coaches, including Andy Reid, Pete Carroll, Ron Rivera and John Harbaugh.
"We encourage players and coaches who are feeling ill, no matter how mild the symptoms, it is best to err on the side of caution," Reid says in the video.
Sills said the PSA was important because virus symptoms are different for the vaccinated, often leading them to miss or dismiss minor signs of infection.
"Symptom reporting is more important than ever," Sills told the AP. "Vaccinated people may have very different symptoms. Last year, for those who had COVID-19, they might have a high fever and chills and trouble breathing ... and with the vaccination they may have only nasal congestion or a mild sore throat or a headache or fatigue."
The physician also said he was much more confident the league will play a full season this year than he was last.