Los Angeles County announced Thursday it will revive an indoor mask mandate applying to everyone regardless of vaccination status in response to rising coronavirus cases and hospitalizations linked to the highly transmissible delta variant.

The order to take effect late Saturday in the county of 10 million people marks the most dramatic reversal of the country’s reopening this summer as experts fear a new wave of the virus.

Officials suspect the delta variant, now estimated to account for half of new infections in the United States, is fueling a resurgence of the virus nationwide. The coronavirus case rate has more than doubled since late June. Average daily deaths have remained under 300 through July, likely due to higher immunization rates among senior citizens, who are more likely to die after contracting the virus.

Los Angeles County reported seven consecutive days of more than 1,000 new infections, which officials said amounts to “substantial transmission.” The daily test positivity rate also has risen, from about 0.5 percent when the county reopened June 15 to 3.75 percent, a measure that suggests more cases in the community are going undetected. Officials also reported nearly 400 hospitalized Wednesday with covid-19, up from 275 the previous Wednesday.

“Masking indoors must again become a normal practice by all, regardless of vaccination status, so that we can stop the trends and level of transmission we are currently seeing,” county officials said in a Thursday newsletter announcing the mandate. “We expect to keep this order in place until we begin to see improvements in our community transmission of covid-19. But waiting for us to be at high community transmission before making a change would be too late.”

The mask mandate, originally lifted June 15, follows a “strong recommendation” by health officials in late June to wear face coverings indoors again while authorities review whether the delta variant can be transmitted by fully vaccinated people. While real-world data suggests all three vaccines authorized in the United States protect against severe illness or death from the delta variant, it’s unclear whether the vaccines would block transmission when a person contracts the virus but does not become ill.

As new coronavirus variants emerge and the longevity of vaccine protection remains unknown, scientists are researching how booster shots could work. (John Farrell/The Washington Post)

About 70 percent of coronavirus samples from Los Angeles genetically sequenced between June 27 to July 3 were identified as delta variants, the county said in a news release. The release justified the mask mandate based on evidence a “very small number of fully vaccinated individuals can become infected and may be able to infect others.”

Los Angeles has above average immunization rates, with 69 percent of people 16 and older receiving at least one dose and 61 percent fully vaccinated. The rates of people with at least one dose are lower among Black and Latino residents, at 45 percent and 55 percent, respectively.

Despite the relatively high overall vaccination rates, Los Angeles County Health Officer Muntu Davis previously told The Washington Post that officials worry the new strain can rapidly spread through the county’s 4 million unvaccinated people, including children who are not eligible, and in communities with lower immunization rates.

Clusters of the virus are erupting nationwide, including in mountain states including Wyoming, Colorado and Utah. States in the Ozarks, such as Missouri and Oklahoma, have seen the number of cases and hospitalizations skyrocket, as have places along the Gulf Coast.

Federal health officials in recent weeks have stood by the Centers for Disease Control and Prevention guidance allowing vaccinated people to go maskless in most situations. But the CDC also said localities should feel free to adopt more stringent rules depending on local conditions.

Some experts raised concerns that mandating masks for vaccinated people sends mixed messages about the effectiveness of vaccines at a time authorities are trying to persuade holdouts that the vaccines work. Others worry there’s no real way of enforcing mask mandates that apply only to the unvaccinated when the United States has not developed a vaccine passport system and businesses rarely ask for proof of vaccination.

Health departments in areas with rising caseloads have largely eschewed new restrictions to prevent transmission. The national vaccination rate has settled at close to 500,000 doses per day, one-sixth of the more than 3 million per day in mid-April. Nearly 3 in 10 Americans say they are not likely to get vaccinated, according to a recent Washington Post-ABC poll.

U.S. Surgeon General Vivek H. Murthy issued a health advisory Thursday, warning that misinformation about covid-19 poses a threat to the nation’s efforts to control the virus and stymies efforts to reach herd immunity through immunization.

“Millions of Americans are still not protected against covid-19, and we are seeing more infections among those who are unvaccinated,” Murthy said at a news briefing.

Dan Keating contributed to this report