The NFL is battling the latest COVID-19 surge with an understanding that symptomatic individuals are driving transmission within the team environment, chief medical officer Dr. Allen Sills said Thursday, with no indications of asymptomatic spread.

That position represents a significant departure from the pandemic-long stance of public health authorities, who have warned about the possibility of individuals spreading the virus without being aware they are infected.

Sills first told the NFL Network that the league has not seen verifiable asymptomatic spread this season, and later fleshed out his point in an interview with ESPN.

"I think all of our concern about [asymptomatic spread] has been going down based on what we've been seeing throughout the past several months," Sills told ESPN. "We've got our hands full with symptomatic people. Can I tell you tonight that there has never been a case when someone without symptoms passed it on to someone else? No, of course I can't say that. But what I can say to you is that I think it's a very, very tiny fraction of the overall problem, if it exists at all.

"Clearly if you want to look at the overall pattern and concern about transmission, it is not being driven by people who have no idea that they are infected and they are infecting scores of others. This is being driven by people with symptoms and the exposures during that symptomatic period."

In response to the Omicron variant, the NFL and NFL Players Association agreed last week to halt weekly testing on vaccinated players and begin random testing of a sample across teams and positions. Vaccinated players who report symptoms are required to be tested, and unvaccinated players continue to be tested daily. The NFL is still performing 1,000 tests per day for players and staff, Sills said.

But the new protocols raised questions about the possibility that infections among asymptomatic, vaccinated players and coaches would go undetected, leading to additional transmission in team facilities. Sills said Thursday night that the NFL and NFLPA had grown confident prior to the rise of Omicron that COVID-19 spread overwhelmingly has come from those with symptoms. The NFL was able to trace, using genomic sequencing, the source of nearly every positive case over that time period and found that approximate 87% of them came from outside team facilities.

"Asymptomatic transmission inside our facilities just didn't fit with what we were seeing," Sills said. Only when the virus "takes hold and starts to replicate" is a person likely to begin spreading it, he said.

"Once it replicates enough to cause you to have symptoms," he added, "now there's enough there for us to say that you're going to be shedding it out in a live form to infect someone else. It's really a matter of degree of how strongly we think you may be infected. You have to get to a point where you're infected enough that you're beginning to shed active viral particles, and that usually correlates with the onset of symptoms."

That information drove the decision to focus on symptom-based testing. Under the previous protocols, Sills said, the league had a number of instances where an asymptomatic person tested negative and then developed COVID-19 symptoms a day or two later. The individuals attributed their symptoms to something other than COVID-19 based on the earlier test result and did not report them.

"We were seeing people walk around for two or three days and expose others because they didn't speak up about their symptoms because they had gotten that negative test," Sills said. "So we're at our best when we're testing at the onset of symptoms."

More than 94% of players and nearly 100% of coaches are vaccinated. A total of 46 players tested positive Thursday, according to ESPN's Field Yates, bringing the total to 154 this week and more than 300 in the past two weeks.

Players must isolate for 10 days when they test positive, but the new protocols give vaccinated players several options to test out sooner based on a combination of negative tests and control threshold (CT) readings, beginning as quickly as the day after their original positive test.