But once the main event begins, it will almost be business as usual, with maskless fans roaming the grounds and sitting next to one another, much as it has been with New York City’s two baseball teams, the Mets and the Yankees.
Dr. Hainline said that some of the strategy behind the relatively relaxed protocols had been derived from monitoring the situations at the two baseball stadiums, which opened to full capacities in June.
Fans without proof of vaccinations will be required to wear masks while eating or shopping indoors and must take food outside to eat.
Dr. Hainline added that even though masks would not be required for unvaccinated fans outdoors, he encouraged those who have not been vaccinated to wear them while at the tournament. But he also knows that not all will and that not all transmission of the virus can be avoided at a huge event like the U.S. Open.
“The goal is not to prevent a single infection,” he said. “The goal is to prevent an outbreak and an uptick, and New York City has remained very steady. And we will continue to monitor and will continue to follow the advice of our health authorities.”
Players who are not vaccinated are encouraged to wear masks everywhere when not practicing or competing. If they come into close contact with someone who has the coronavirus, they will be required to go into quarantine. Vaccinated players may not have to go into quarantine after such contact, depending on recommendations from tournament physicians. That means an unvaccinated player who hasn’t tested positive, but who has been in close contact with someone who has, may have to withdraw from the tournament.
The U.S.T.A. said it was still gathering data on how many players had been vaccinated, but Dr. Hainline indicated that number was well below the 85 percent rate he said that N.C.A.A. student-athletes in all sports had achieved. Last week, Stefanos Tsitsipas, the third-ranked player on the men’s tour, said he had not been vaccinated and did not see a reason for people his age (he is 23) to be.