Covid postponements are roiling the sports world.

Facing an alarming increase in athletes testing positive for the coronavirus, U.S. professional sports leagues have made a host of changes to schedules and health rules in lieu of stopping their seasons.

The N.B.A. announced Sunday that it would postpone five games, bringing to seven the total number of Covid-related postponements in the N.B.A. this season. And the N.H.L. announced that because of “concern about cross-border travel” and changing federal travel rules, it would postpone 21 games pitting Canadian against U.S.-based teams. Those games, which had been slated for this week, are scheduled to resume beginning on Dec. 27.

On Saturday, the N.F.L. and N.F.L. Players Association changed their testing protocols, the fourth such adjustment in a week. In a memo to all 32 teams, Commissioner Roger Goodell said that players will be given at-home tests to use before entering league facilities.

Vaccinated N.F.L. players who are asymptomatic will be subject to “strategic and targeted” tests, while players who report symptoms will be tested “promptly.”