After the league reported a season-high of 36 positive cases in a single day on Monday, the NFL is requiring all Tier 1 and 2 employees to get a COVID-19 booster shot by Dec. 27.

A memo issued to all 32 clubs reiterated the league’s stance on being vaccinated as the "most effective" method of being protected against COVID. 

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"Our own data has shown that unvaccinated players have contracted the virus and lst time as close contacts at a much greater rate than vaccinated players," the memo, obtained by the NFL Network, read. 

Aaron Rodgers Green Bay Packers NFL football

Aaron Rodgers of the Green Bay Packers walks across the field in the second quarter against the Tampa Bay Buccaneers during the NFC Championship game at Lambeau Field on Jan. 24, 2021 in Green Bay, Wisconsin.  (Dylan Buell/Getty Images)

"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," the memo continued. "In accordance with the CDC recommendation, all Tier 1 and 2 staff, who have previously been vaccinated and are eligible under the CDC guidelines, must receive a booster shot no later than December 27."

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Under the new policy, any employee that is six months out from their initial Pfizer or Moderna shot or two months out from a Johnson and Johnson shot, must receive a booster shot. Failure to comply may lose their tiered status. 

Green Bay Packers quarterback Aaron Rodgers leads the team onto the field during a game between the Green Bay Packers and the Seattle Seahawks at Lambeau Field on Nov. 14, 2021 in Green Bay, WI.

Green Bay Packers quarterback Aaron Rodgers leads the team onto the field during a game between the Green Bay Packers and the Seattle Seahawks at Lambeau Field on Nov. 14, 2021 in Green Bay, WI. (Patrick McDermott/Getty Images)

The announcement came the same day that a season-high 36 players were placed on the league’s COVID-19 list. Among the players added, were Jalen Ramsey and Tyler Higbee, who missed the Los Angeles Rams’ 30-23 win at Arizona that tightened the NFC race.

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Jalen Ramsey #5 of the Los Angeles Rams runs on to the field to warm up for the game against the Tennessee Titans at SoFi Stadium on Nov. 7, 2021 in Inglewood, California.

Jalen Ramsey #5 of the Los Angeles Rams runs on to the field to warm up for the game against the Tennessee Titans at SoFi Stadium on Nov. 7, 2021 in Inglewood, California. (Jayne Kamin-Oncea/Getty Images)

For now, players have not been required to receive a booster shot. 

The Associated Press contributed to this report.