LOS ANGELES — With the highly contagious delta variant of the coronavirus continuing to spread statewide, the Los Angeles County Department of Public Health is recommending that all residents wear masks in public indoor spaces — regardless of whether they’ve been vaccinated for COVID-19.

Monday’s announcement is one of the clearest signals yet of just how seriously health officials are taking the strain, and the danger it poses, particularly to those who have yet to be inoculated.

Officials have said the delta variant does not pose a risk to vaccinated people. But there is growing concern for those who have not been vaccinated and are at higher risk. As of last week, 3 in 5 Californians, or 60.5%, have now received at least one dose of COVID-19 vaccine.

While not a new mask mandate, the county is urging that, as a precaution, “people wear masks indoors in settings such as grocery or retail stores; theaters and family entertainment centers, and workplaces when you don’t know everyone’s vaccination status.”

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“Until we better understand how and to who the delta variant is spreading, everyone should focus on maximum protection with minimum interruption to routine as all businesses operate without other restrictions, like physical distancing and capacity limits,” officials wrote in a statement.

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They added, however, that “fully vaccinated people appear to be well protected from infections with delta variants.”

Officials and experts say the variant, which was first identified in India and is also known as B.1.617.2, may be twice as transmissible as the conventional coronavirus strains.

It’s now the third-most common variant in California — accounting for 14.5% of coronavirus cases analyzed so far in June, up from 4.7% in May.

L.A. County had confirmed 123 delta variant cases as of last week, 49 of them among residents of Palmdale and Lancaster. Fourteen cases of the delta variant were in people from a single household.

“While COVID-19 vaccine provides very effective protection, preventing hospitalizations and deaths against the delta variant, the strain is proving to be more transmissible and is expected to become more prevalent,” county Public Health Director Barbara Ferrer said in a statement. “Mask wearing remains an effective tool for reducing transmission, especially indoors where the virus may be easily spread through inhalation of aerosols emitted by an infected person.”

At this point there is no widespread scientific consensus as to whether the delta variant is more likely to cause more serious illness than other strains.

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(Times staff writers Rong-Gong Lin II and Alex Wigglesworth contributed to this report.)