Mayer, the union’s medical director, disagreed.
“You can’t intervene in the face of what you know about the virus unless you know what the virus is doing,” he said.
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Dr. Allen Sills, the N.F.L.’s chief medical officer, said in an interview on Nov. 30 that the rise in cases this season needed to be examined and responded to in context of the current landscape.
The N.F.L. collects real-time data from testing, contact tracing and genomic sequencing, essentially virus fingerprinting that enables scientists to spot variants and map out chains of transmission. The genomic sequencing, Sills said, indicates that in the majority of cases, transmission took place in the outside community, not the highly vaccinated team population.
“We don’t think Covid Zero is an achievable goal,” he said, especially with players and their families interacting with the outside world more than they did last season, when stay-at-home orders limited movement. The mission now, Sills said, is avoiding outbreaks, severe disease and cardiac complications.
“I think that our story this year has shown that while we will still have positive tests, we’re not seeing the same disease burden that we saw in 2020,” he said.
The N.F.L.’s testing showed that vaccinated players who test positive have tended to have milder and shorter illnesses — with approximately 20 percent testing out of isolation before 10 days, the league said. Unvaccinated players must quarantine for at least 10 days if they test positive, while the vaccinated may return when they produce two negative tests 24 hours apart.
Dr. Michael Osterholm, an epidemiologist at the University of Minnesota and a league consultant who served on President Biden’s Covid-19 advisory board, along with other medical experts who review the N.F.L.’s data weekly, said they believed the current protocols were sufficient so far to contain coronavirus spread. When cases or exposures arise, clubs temporarily follow a set of stricter protocols, which require more remote meetings, masking and daily testing for all personnel, regardless of vaccination status.