He pointed to the Lollapalooza music festival over the summer in Chicago as evidence.
“It was an incredibly crowded gathering, and everyone in that context was required to be vaccinated or have one negative test,” Dr. Jha said. “We have evidence that there was little to no spread.”
But Dr. Jha did caution Mr. de Blasio to give himself an escape clause should cases increase sharply in the days before New Year’s Eve, requiring the return of the celebration to be pushed to next year.
Some experts pointed out that the risk might not be contained within Times Square. People will also have to consider what happens before and after the ball drop, with people going in and out of nearby bars and restaurants to eat, warm up and use the restroom. But New York City, unlike many locales, requires those eating and drinking indoors to provide proof of vaccination.
Some major restrictions against international travelers were recently eased by the United States, so the ball drop is likely to attract revelers from around the world, drawing together people from areas of both lower and higher rates of the virus.
Denis Nash, a professor of epidemiology for the City University of New York Graduate School of Public Health & Health Policy, said that although the vaccination requirement meant the risk of transmission at the event would be greatly reduced, those who travel from outside the city to attend the ball drop should consider the possibility of bringing the virus back to their hometowns.
“There will be people coming from places that don’t have much Covid going on right now,” Mr. Nash said. “We need to be thinking about seeding, outbreaks and spread, not just in our own backyards but everywhere.”
Given the safety guidelines, attending the ball drop would not be an irrational choice, Mr. Nash said. But people will have to weigh their individual comfort levels and the amount of risk they may pose to others, he said, advising revelers to wear a mask the entire time.