The NFL bent over backward to accommodate teams in the 2020 season as COVID-19 ravaged the league and the country. Games ended up being played every day of the week to ensure that the season was played in full when the vaccine was not widely available.
That will not be the case this season. The league will enforce forfeits if a game is canceled due to a breakout among unvaccinated players. The league does not currently mandate vaccines, but there are incentives for players to get vaccinated.
According to the New York Times, close to 90% of the NFL has been vaccinated, as the Delta variant sweeps the nation.