As part of the government’s Coronavirus Aid, Relief, and Economic Security (CARES) Act that offers financial support to United States airlines, the airlines must continue to serve the airports they did before March 1, as long as it is “reasonable and practicable,” but routing can change, international destinations are not included, and exceptions are granted. Airlines serving different airports within one metro area can consolidate service to one of those airports.

The frequency of the required flights is determined by the airlines’ schedule before March. If an airline was providing at least one flight a day at least five days per week, it now is required to provide at least one flight per day, five days per week, for that area. If the area was served fewer than five days per week, the airline needs to service it once a week. But if too few passengers book tickets on a flight or no one shows up at the gate, the flight can be canceled.

The airlines can change routing, so cities that may have only recently gained nonstops are losing them, and passengers hoping to travel from those cities may have to fly circuitous routes that go through an airline’s hubs.

United changed about 130 nonstops to connecting flights through one or even two of its hubs. Appleton, Wis., a small city in the northeast part of the state, got its first nonstops to Denver in June 2018. Now, fliers are being offered a route that goes from Denver south to Atlanta, back north to Chicago, and then on to Appleton. The short United Airlines flight from Los Angeles International Airport to Palm Springs will now go through San Francisco or Denver. Delta’s former nonstop between Raleigh-Durham, N.C., and Seattle now flies through Detroit, Minneapolis or Atlanta.

Some of the suspended nonstop flights may still appear on the companies’ websites, but these will be updated soon.

Kathy Osborne decided on March 30 to leave New York for Fort Myers, Fla., where her parents own a home, and purchased a ticket for April 4. “We wanted to get out of there,” said Ms. Osborne of her attempt with her fiancé to leave town. “It was truly scary because people in our neighborhood didn’t seem to be taking the quarantine seriously at all,” she said.

Her first reservation with United Airlines was canceled the day after she purchased it, and the airline said she wouldn’t be able to fly for two weeks. The airport at Fort Myers has dropped to about 85 arrivals per day, down from about 230 per day, over the last month, according to Flightradar24.