The next few Calgary Flames games are on hold after 27 players and staff were sidelined due to COVID exposure. The Brooklyn Nets had only eight players available for Tuesday night's game after nearly half the team had to enter the league's COVID protocols – two of them just an hour before tipoff. And in the past three days, nearly 100 NFL players and some staff have tested positive – including both the starting quarterback and head coach of the Cleveland Browns. Make no mistake: COVID-19 is once again surging in America's pro sports leagues. "We are seeing a fairly fast and sudden increase over multiple markets and multiple sports and multiple countries," said Zach Binney, a sports epidemiologist at Oxford College of Emory University. "I think everybody's trying to figure out what's happening now, what's driving this?" With schedules disrupted and stars sidelined, sports leagues are rethinking safety protocols as experts work to understand whether the new outbreaks were being caused by waning vaccine efficacy, looser vigilance around Thanksgiving, or the tip of an omicron iceberg – or perhaps a heady combination of all. In the NFL, dozens of players tested positive on Monday and Tuesday alone. The Washington Football Team alone reported 15 players out. On the Cleveland Browns, 13 sidelined players include star quarterback Baker Mayfield. The Browns held their Tuesday meetings virtually, said head coach Kevin Stefanski. (Stefanski, who said Monday that he had previously received a booster shot, has now also tested positive and could miss Saturday's game.) Players and staff inside team facilities must wear masks regardless of vaccination status and are subject to daily testing. With just four weeks left in the regular season and playoff hopes on the line, even teams who have so far escaped this week's outbreak say they will tighten safety measures. "We've elevated our alerts about mask-wearing and all the stuff we can do there," said Seattle Seahawks coach Pete Carroll Monday. "Everybody's on high alert." One worry is the omicron variant, which currently makes up about 3% of cases in the U.S., according to the Centers for Disease Control and Prevention. But Binney told NPR that because samples take time to process, the CDC variant data often lags. "So we're really peering into the past," he said. Early data about omicron suggest it may be more transmissible than previous variants. "If one person in an organization gets unlucky enough to get omicron, then goes in and they're spending a lot of time unmasked indoors around folks with what we know about the transmissibility of omicron – suddenly, you've got a huge pile of omicron cases," Binney said. According to early reports, most of the positive cases among sports leagues in the past week have been mild or asymptomatic. Though none of the major professional sports leagues in the U.S. has required players to be vaccinated, vaccination rates are much higher than the general public. About 72% of Americans aged 18 or older are fully vaccinated, according to the CDC. Meanwhile, more than 94% of NFL players are vaccinated, the league says. The NBA has reported a vaccination rate of about 97%. And in the NHL, only four players out of nearly 700 on team rosters were not vaccinated, the league's commissioner said in October. All three leagues have different safety protocols for players who are vaccinated and those who are not, allowing those who have received the shots to wear masks less often, travel more freely and dine together indoors. Now, leagues are looking to strengthen those measures. In a memo to teams Monday, the NFL said it would require coaches, staff and other team personnel to receive a booster shot by Dec. 27, with limited exceptions. It notably did not make the same requirement of players, as those rules are subject to negotiation with the players' union. "We are currently in talks with our player membership and the league, exploring ways to update our protocols for the health and safety of our members," said Brandon Parker, a spokesman for the NFL Players' Association. The NFLPA has already urged the league to return to daily testing. (The league currently conducts most of its tests once a week each Monday, with follow-up tests triggered if a player tests positive.) In hockey, the NHL will require stricter protocols for all teams and players through Jan. 7, the league told teams Wednesday. That reportedly includes more masking, restrictions on indoor dining and more frequent tests.And in the NBA, players eligible for the booster shot must receive one by this Friday or else face stricter safety protocols, including game-day testing, the league has announced. About 60% of players have already received one. Across the U.S., reported cases have trended up over the past month, with a current daily average of 117,364 cases per day. "I do worry that it's possible – maybe not likely, but possible – that these sports leagues' numbers are a bit of a canary in the coal mine for the rest of us," Binney said. For those watching at home, Binney says that fans would do well to follow the lead of the sports leagues – which, despite the recent outbreaks, have had success in vaccinations and booster shots, using regular rapid testing and following stricter safety measures upon exposure to COVID, he said. Copyright 2021 NPR. To see more, visit https://www.npr.org.