An unusually high number of positive COVID-19 tests on Monday has the NFL and NFL Players Association considering further changes to the league's coronavirus protocols as the regular season enters its final four weeks.

Sources told ESPN's Adam Schefter that the league had 37 positive COVID-19 tests on Monday. These numbers tend to be higher on Mondays because that's when most teams test their vaccinated players and personnel (who have to be tested only once a week, unlike unvaccinated players, who test daily). But to put Monday's number into perspective, consider that the league had only 72 positive tests in a two-week stretch from Nov. 14 to Nov. 27.

The NFL sent a memo to its teams on Monday stating that all Tier 1 and Tier 2 employees, excluding players, who are eligible to receive a COVID-19 vaccination booster are mandated to receive the shot no later than Dec. 27.

"On November 29, the CDC issued a study showing that the effectiveness of the approved COVID-19 vaccines may decrease over time and has recommended that all eligible vaccinated individuals over the age of 18 should receive a booster shot. Given the increased prevalence of the virus in our communities, our experts have recommended that we implement the CDC's recommendation," the memo, which was obtained by ESPN, stated.

One source told ESPN that Monday's positives included one case of the Omicron variant in Washington and that at least five teams -- the Atlanta Falcons, the Detroit Lions, the Minnesota Vikings, the Chicago Bears and Washington -- have entered the league's enhanced COVID-19 protocols, which require daily testing regardless of vaccination status, mandatory masking at team facilities and other stringent measures.

The positive case of the Omicron variant in Washington was a Tier 3 employee who had limited contact with other members of the organization, sources told Schefter.

The NFLPA has continued to push for daily testing for all players and personnel, regardless of vaccination status, as a means of identifying and tracking cases earlier and preventing spread in team facilities. The league has resisted the union's calls for daily testing, saying resources can be better directed at different parts of the COVID-19 fight and that daily testing could lead to a false sense of security and less strict adherence to the other protocols.

It's unclear what sorts of changes to the protocols the league and union might make, as those discussions were just beginning in light of Monday's news. Expecting a surge in positive cases following the Thanksgiving holiday, the league and union did put in place stricter measures for the week that immediately followed Thanksgiving.

ESPN's Dianna Russini contributed to this report.