Fully vaccinated people traveling in buses, trains and airplanes must continue to wear a mask, the Centers for Disease Control and Prevention said Thursday, even as it said they could go without one in most other indoor settings.

CDC Director Rochelle Walensky said the agency is continuing to review its travel policies, but she did not explain the reasoning during a briefing Thursday on the new recommendations.

“Right now for travel we are asking for people to continue to wear their masks,” she said.

As drug companies expand coronavirus vaccine access to children, officials are working to persuade people who are skeptical or unwilling to get vaccinated. (Luis Velarde, Jonathan Baran, Erin Patrick O'Connor/The Washington Post)

The Biden administration issued an order in January requiring masks for public transportation, including transit stations and airports. At the end of April, the Transportation Security Administration extended enforcement of the rules through Sept. 13.

TSA spokeswoman Alexa Lopez said Thursday that while the agency’s order remains in force, “we will continue to work closely with the CDC to evaluate the need for these directives.”

The CDC advised against all travel through much of the pandemic, but last month it said taking a trip poses a low risk to people who have been vaccinated.

Airplanes in particular have become battlegrounds over mask enforcement, with the Federal Aviation Administration stepping in to levy hefty fines on disorderly passengers. When the TSA extended its order, Sara Nelson, president of the Association of Flight Attendants, said it would help keep passengers and aviation workers safe.

While it’s not clear that airplanes present a greater risk of transmission than other indoor spaces — some studies suggest they are actually safer — requiring masks in planes could help enforcement of the policy, especially as flight attendants have no reliable way to know which passengers have had their shots.

For that reason, Jennifer Nuzzo, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health, said she expects mask-wearing to continue in transportation settings, as well as some other places open to the public, such as grocery stores.

“I really don’t expect public settings to change that much until case numbers come down,” she said.

The new CDC guidance says fully vaccinated people are at low risk of becoming sick with covid-19 or spreading the coronavirus to others. For that reason, the agency said people generally don’t need to wear masks or follow social-distancing protocols, a significant step toward resuming pre-pandemic life.

The agency’s guidance to the public has evolved as it digests new data.

On Wednesday, the CDC said passengers on cruise ships, which became notorious for virus outbreaks early in the pandemic, do not need to wear masks outdoors unless they are in a crowd. The agency said that recommendation is consistent with its other mask policies.

International travel continues to be subjected to stricter rules, with visitors banned from many nations, including much of Europe and India. Fully vaccinated people coming to the United States from other countries must still get tested within three days of boarding a flight and are advised by the CDC to get tested three to five days after arriving.

In the Washington area, a spokesman for the region’s Metro transit system noted that the transportation mask mandate is a federal order and reiterated that masks are still required in buses and rail cars.