“It’s low, but there is no zero risk situation on an airplane,” said Dr. Aisha Khatib, the chair of a group focused on responsible travel for the International Society of Travel Medicine, adding that the risk goes up in a surge situation and while getting on and off planes if the ventilation system is not on. “Masks will definitely decrease your risk of transmission,” she said.

Canada, where Dr. Khatib lives, is grappling with similar situation, she said. Masks are still required on airplanes and in airports there, but as an expiration date approaches for Canada’s mask mandate, cases have been rising in some parts of the country.

By many accounts, enforcement has been one of the most challenging aspects of the mask mandate in the United States, with many passengers verbally and even physically assaulting flight attendants who reminded them to cover their nose and mouth. In 2021, more than 4,000 mask-related incidents were reported to the Federal Aviation Administration.

Ahead of the decision, major unions representing flight attendants and Transportation Security Administration employees, two groups that have to deal with enforcing the rule, declined to take a stance.

“Whatever the agency puts in place, we have to comply with it,” said Hydrick Thomas, the president of the union that represents T.S.A. employees, on Tuesday. He added that he believes that masks protect his employees, their families and “the flying public.”

The extension provoked applause from some travelers and commuters, with some arguing that it should remain in place even longer.

“The C.D.C. is extending the mask mandate for public transport for two weeks,” Dr. Lucky Tran, a scientist and activist who was one of the organizers of the March for Science in 2017, wrote on Twitter. “That’s not enough. Millions rely on public transportation every day to get to work or access essential services. While we are in a pandemic, we need mask mandates to keep society open and accessible to all.”