(NewsNation Now) — The NFL is requiring players and key personnel to receive a booster shot of the COVID-19 vaccine by Dec. 27, according to a league memo. The decision comes as COVID-19 outbreaks have been affecting game schedules for multiple major sports leagues.

Tier 1 and 2 personnel who previously have been vaccinated must receive a booster shot within the two-week window. Tier 1 staff consists of players, coaches and trainers. Tier 2 includes general managers and front-office employees.

The mandate covers those who received the Pfizer or Moderna shot more than six months ago and those who received a Johnson and Johnson shot more than two months ago.

As of this week, 97 percent of National Basketball Association (NBA) players have had at least one shot, and 60 percent report receiving a booster dose.

In the National Hockey League, nearly all players are vaccinated except for one, Detroit Red Wings forward Tyler Bertuzzi.

As for Major League Baseball, 86 percent of Tier 1 employees are vaccinated, according to the Wall Street Journal.

None of the associations policies requiring players to be vaccinated or get fired, but all have various COVID-19 testing procedures if individuals are unvaccinated.

The NBA has said it will require players who don’t have a booster to be tested on game day and be subject to additional tests. The NHL is not mandating booster shots, but it has canceled holiday parties and public engagements over the next several weeks.