Vaccination cards joined funny hats and noisemakers as the must-have accessories for Friday night’s scaled-back New Year’s Eve celebration in Times Square.

Mr. Biden was himself exposed to the coronavirus late last week but has tested negative, White House officials said on Monday. With Americans already jittery about their holiday plans, he will try to reassure the nation while reminding people, yet again, that their best defense against Covid-19 is to get vaccinated and, for those who are eligible, to get booster shots.

The preliminary assessment says the variant’s mutations could make it more transmissible and better able to evade the body’s immune defenses, but many questions remain.