An indoor mask mandate returns to Alameda County, Calif., home to Oakland.

Alameda County, the San Francisco Bay Area’s second-most populous county, reinstated on Friday a mask mandate in most indoor spaces, which it had dropped in February.

Officials cited growing hospitalizations in Alameda, whose county seat is Oakland, which are up about 35 percent over the last two weeks to a daily average of 129, as of Thursday, according to federal data. Nationally, about 28,000 people were currently in hospitals with the coronavirus, an increase of 17 percent over the last two weeks.

Alameda is the first county in California, and the largest jurisdiction in the United States, to issue a universal indoor mask order since the end of the winter Omicron surge.

Further south, the public health director of Los Angeles County said Thursday an indoor mask mandate could be reintroduced later this month if cases continue to rise. In April, Philadelphia became the first major American city to reinstate an indoor mask mandate in response to rising coronavirus cases, though it was dropped after a few days.

“Daily new admissions of patients with Covid-19 rapidly increased in recent days and now exceed last summer’s peak,” the Alameda County Public Health Department said in a statement on Thursday announcing the new rules.

The order requires masks to be worn at indoor businesses and workplaces, including offices, stores, theaters and conference centers; in restaurants and bars when not eating or drinking; and on public transportation, including taxis and ride-share vehicles.

The county, with its 1.6 million people in the eastern Bay Area, is not mandating masking in schools, with the academic year in its final days, and the order does not apply to Berkeley, which has its own public health department.

“Rising Covid cases in Alameda County are now leading to more people being hospitalized and today’s action reflects the seriousness of the moment,” Dr. Nicholas Moss, the county health officer, said in the statement.

“We cannot ignore the data, and we can’t predict when this wave may end,” he said. “Putting our masks back on gives us the best opportunity to limit the impact of a prolonged wave on our communities.”

Oakland International Airport will also require everyone two and older to wear masks indoors starting Friday. In April, a federal judge in Florida struck down a mask mandate on public transport, which also applied to airports, train stations and other transportation hubs. The country’s largest airlines stopped requiring masks on flights, ending a practice that most carriers had followed for nearly two years.

There has not been a widespread return to mandates in California, although some schools and universities have reinstated them. The Sacramento City Unified School District is mandating masks for all staff and students beginning on Monday. UCLA resumed indoor masking last week, and Cal Poly San Luis Obispo did the same on Tuesday.

In Los Angeles County, the nation’s most populous county with about 10 million people, Barbara Ferrer, the public health director, said Thursday that a mask mandate could return “if we continue on the current trajectory.” Hospitalizations are rising even faster there than in Alameda, averaging about 400 each day as of Thursday, a 62 percent increase over the last two weeks, according to federal data.

Alain Delaquérière contributed research.