Los Angeles County’s jump in coronavirus cases continued Saturday as a new rule requiring masks in public indoor settings was set to take effect, a restriction officials hope will slow the spread of the virus among the unvaccinated.

The county is by far the biggest jurisdiction in the nation to require masks again. But with COVID-19 cases rising across the nation largely because of the highly infectious Delta variant, officials elsewhere will be watching to see if the effort works.

People who have received vaccinations are extraordinarily protected from the coronavirus, including the Delta variant. But officials are asking them to wear masks indoors as a way of also forcing unvaccinated people — who are at risk — to do the same. About 52% of L.A. County residents are fully vaccinated, and roughly 60% have gotten at least one shot. But given the region’s enormous population, that still leaves millions vulnerable.

“Given the increased intermingling among unmasked people where vaccination status is unknown, the millions of people still unvaccinated, and the increased circulation of the highly transmissible Delta variant, we are seeing a rapid increase in COVID-19 infection,” county Public Health Director Barbara Ferrer said in a statement.

Dr. Robert Kim-Farley, a medical epidemiologist and expert on infectious disease with the UCLA Fielding School of Public Health, said he’s not worried about the pandemic approaching the same level of devastation seen half a year ago, but is “concerned about the trajectory and the speed of the doubling of new cases.”

“Hopefully, this will be the wakeup call for those who are still vaccine hesitant to roll up their sleeves and get vaccinated,” he said.

Under L.A. County’s order, masks will be required to be worn in all indoor public settings, such as theaters, stores, gyms, offices and workplaces, and in restaurants when not eating and drinking. Those exempted include children younger than 2. Indoor dining is still allowed, but patrons are asked to wear masks when they are not eating or drinking.

Other California counties have urged people to wear masks indoors, but none have followed L.A. and mandated the practice.

Los Angeles County on Saturday confirmed 1,827 new coronavirus cases — the second-largest daily total the region has seen in months — and an uptick that public health officials are saying is evidence of an alarming trend of increased community spread.

Saturday’s case count was four times higher than the 457 daily infections reported on July 4 and eight times higher than the 210 cases the county had on June 15 when California fully reopened, health officials said. The county also reported 11 new related deaths.

Last month, the county was seeing as few as 161 new coronavirus cases a day — the lowest rates since the first few weeks of the pandemic. But daily cases have been climbing increasingly rapidly since California fully reopened on June 15.

On July 9, the county began a streak of consecutive days with at least 1,000 cases a day reported — the numbers hit 1,059 on Monday, 1,103 on Tuesday, 1,315 on Wednesday, 1,537 on Thursday and 1,902 on Friday.

Hospitalizations are also worsening. On Friday, there were 507 people with COVID-19 in L.A. County hospitals, the highest number since April 14, and more than double the number over the last month. On a per capita basis, that means there are now more than 5 hospitalizations for every 100,000 residents in the county — a figure some experts say could be a reasonable threshold to reimplement masking policies indoors.

National officials and experts have begun to endorse the county’s move, given the sharp rise in cases, to require masks by everyone indoors, regardless of vaccination status, a mandate set to take effect Saturday at 11:59 p.m.

“Under certain circumstances, when you have a high level of dynamics of infection, be that in Los Angeles or wherever, the local authorities do have the discretion of going that extra mile, of going the extra step it takes to make sure that the spread of this virus is really contained, and they do that by saying that everyone should wear a mask,” Dr. Anthony Fauci, the U.S. government’s top infectious-disease expert, said Friday on NBC’s “Nightly News.”

Dr. Ashish Jha, dean of the Brown University School of Public Health, told ABC’s “World News Tonight” that he thought L.A. County was taking appropriate action, as were other California counties in the San Francisco Bay Area and the Sacramento area that are recommending mask use by everyone indoors, regardless of vaccination status.

“These counties are acting prudently, because infection numbers are rising so quickly,” Jha said. “We can’t tell who’s vaccinated and who’s not. And we do know that there are breakthrough infections even for vaccinated people. So it makes sense in places with high rising infection numbers to put the mask mandates back in.”

The vaccines are extraordinarily protective against severe disease and death. Between Dec. 7 and June 7, unvaccinated people in L.A. County accounted for 99.6% of its coronavirus cases, 98.7% of COVID-19 hospitalizations and 99.8% of deaths.

But rising community transmission levels can also raise the risk of “breakthrough infections,” coronavirus infections of vaccinated people. It’s not particularly likely, but it can happen — and is more likely to happen if vaccinated people are close to contagious, unvaccinated people.

“It is a bummer to get a breakthrough infection,” said Dr. Peter Chin-Hong, an UC San Francisco expert in infectious disease. Those who get them have to notify their close contacts; they have to stay home from work — “it’s a disruption to life.”

That’s why Chin-Hong said he thinks it’s a good idea for even vaccinated people to “just put on your mask if if you’re in a crowded indoor setting with a bunch of people you don’t know.”

The good news is that vaccinated people are far more protected against severe illness and death. Nationally, unvaccinated people make up 97% of patients in the hospital now for COVID-19, according to Dr. Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention.

L.A. County’s massive public hospital system has not had to hospitalize anyone for COVID-19 who has been fully vaccinated, Health Services Director Dr. Christina Ghaly said.

Officials hope more masking will help contain the hypercontagious Delta variant of the coronavirus, which is easily spread and has been blamed for increased infections across the country.

“The alarming increases in cases, positivity rates and the increase in hospitalizations signals immediate action must be taken to slow the spread of COVID-19. Otherwise, we may quickly see more devastating illness and death among the millions of residents,” said L.A. County Health Officer Dr. Muntu Davis.

Health officials continue to urge everyone who is eligible to be vaccinated to get their shots immediately. The risk of increased spread is highest among unvaccinated individuals. Cases are rising the most in adults age 18 to 49, county data show.

Officials warn that the more COVID-19 spreads, the more opportunities it has to mutate and potentially create another easily transmittable and dangerous variant.

“The urgency to get more people vaccinated remains high with this level of spread,” Davis said. “For everyone [who is] eligible and still waiting to get vaccinated, the time do it is now.”

While other counties in Southern California are also seeing an uptick in cases, L.A. County is the first to broaden mask requirements.

In Orange County, public health officials have encouraged unvaccinated residents to avoid large crowds and poorly ventilated spaces and continue masking in indoor public areas. However, there are no plans there to implement a new mask mandate.

“With the recent reopening of the state’s economy, we had expected to see an increase in our COVID-19 cases and positivity rates,” said Dr. Clayton Chau, Orange County’s health officer. “This means we need to continue being proactive about protecting our loved ones and neighbors and taking the necessary steps to help reduce the risk of infection throughout our county.”