The NFL told teams in a memo Thursday that if a game is canceled during the upcoming season because of a coronavirus outbreak among unvaccinated players or staff members, and cannot be rescheduled, the team with the outbreak will be forced to forfeit the game.

“If a game cannot be rescheduled within the current 18-week schedule and is cancelled due to a Covid outbreak among non-vaccinated players on one of the competing teams, the club with the outbreak will forfeit the contest,” the NFL said in its memo, a copy of which was obtained by The Washington Post. “ … The forfeiting team will be credited with a loss and the other team will be credited with a win.”

The league delivered its memo — and forfeit threat — as teams begin to report to training camps. The NFL has not required players to be vaccinated, instead relying on education and incentives to encourage vaccinations. But protocols have been relaxed for vaccinated players, and the potential competitive implications were underscored with Thursday’s memo.

According to a person familiar with the situation, 78 percent of NFL players have received at least one vaccine dose, and 14 of the 32 teams have more than 85 percent of their players vaccinated. No team is below the 50 percent threshold, according to that person.

Arizona Cardinals wide receiver DeAndre Hopkins questioned the approach, writing on Twitter: “Never thought I would say this, [b]ut being put in a position to hurt my team because I don’t want to partake in the vaccine is making me question my future in the [NFL].”

Other players, including Washington Football Team defensive end Montez Sweat and Buffalo Bills wide receiver Cole Beasley, have publicly expressed their wariness about vaccinations in recent weeks.

A few teams’ training camps are underway, with most teams scheduled to open Tuesday.

“The league will make every reasonable effort, consistent with underlying health and safety principles, to complete the full 272-game regular season within the current 18 weeks and all postseason games as scheduled, in a safe and responsible way,” the NFL wrote. “ … We do not anticipate adding a ‘19th week’ to accommodate games that cannot be rescheduled within the current 18 weeks of the regular season.”

A team that forfeits a game also could be subject to additional penalties imposed by NFL Commissioner Roger Goodell, according to the memo, “particularly if the Covid outbreak is reasonably determined to be the result of a failure by club personnel to follow applicable protocols.”

The NFL said in its memo that, as with last season, games will be postponed only for medical reasons, not over competitive issues.

“In light of the substantial roster flexibility in place for the 2021 season, absent medical considerations or government directives, games will not be postponed or rescheduled simply to avoid roster issues caused by injury or illness affecting multiple players, even within a position group,” the league wrote.

The NFL played a full 2020 season amid the pandemic, with numerous games being postponed but none being canceled entirely. Two rescheduled games were played on Tuesdays. Another, between the Baltimore Ravens and Pittsburgh Steelers, was played on a Wednesday. Some teams played games at competitive disadvantages because of positive coronavirus tests and contact-tracing quarantines, as when the Denver Broncos lost a game while playing without any of the quarterbacks on their roster.

League officials discussed the possibility of adding a week to the 2020 regular season if it had been needed to accommodate any rescheduled games, but that didn’t happen. Thus, the forfeit threat made Thursday by the league would not have applied to any games last season.

The NFL hopes this season will be played under far less trying circumstances. All 32 teams have been cleared by local authorities for full stadium capacities for games. Vaccinated players are to be tested for the coronavirus once every two weeks and are not subject to quarantines through contact tracing, under protocols developed by the league and the NFL Players Association, while unvaccinated players remain subject to daily testing, contact-tracing quarantines and many of the restrictions that were in place last season.

The league has said that coaches and team staffers must be vaccinated to maintain their ability to work in proximity to players. “Nearly all clubs have vaccinated 100 percent of their Tier 1 and 2 staffs,” the NFL said in Thursday’s memo.

“These operating principles are designed to allow us to play a full season in a safe and responsible way and address possible competitive or financial issues fairly,” the NFL wrote. “While there is no question that health conditions have improved from last year, we cannot be complacent or simply assume that we will be able to play without interruption — either due to Covid outbreaks among our clubs or outbreaks that occur within the larger community.”

If a game is canceled and not rescheduled, neither team’s players would receive their salaries for that week, according to the memo.