LOS ANGELES—One month after California fully reopened for business, many of the state’s most populous counties are putting their masks back on.

Seven counties in the Bay Area issued recommendations that residents wear masks indoors, even if they are fully vaccinated, citing rising Covid-19 infections and the rapid spread of the Delta variant. Two Northern California counties—Sacramento County and neighboring Yolo County—took the same step earlier this week. Los Angeles County on Thursday made masks a requirement in most indoor public spaces starting late Saturday.

The Southern Nevada Health District issued a similar recommendation Friday for crowded indoor public spaces, including casinos and grocery stores.

In a joint statement Friday, the seven Bay Area counties—San Francisco, Alameda, Contra Costa, Marin, San Mateo, Santa Clara and Sonoma, plus the city of Berkeley—said the explosive growth of the Delta variant across the state and the country made masks necessary again.

The Delta variant, which was first detected in India late last year, is estimated to be at least 50% more transmissible than the Alpha variant, or B.1.1.7, which had been dominant in the U.S. since March.

Last month, 43% of newly reported Covid-19 infections in the state were from the Delta variant, the statement said. Now, that figure is up to 58%.

A restaurant in San Francisco in May when the state was beginning to reopen.

A restaurant in San Francisco in May when the state was beginning to reopen.

Photo: David Paul Morris/Bloomberg News

Though infections in the Bay Area, which has one of the highest vaccination rates in the country, remain relatively low, they are rising. Asking vaccinated people to mask was an “extra precautionary measure for all,” the statement said.

Covid-19 vaccines available in the U.S. protect against the Delta variant, but the virus is of great risk to people who aren’t vaccinated, according to public-health and infectious-disease experts.

“After vaccination, masking is the next most powerful tool we have to protect ourselves and each other during this latest wave of infections,” said Alameda County Health Officer Dr. Nicholas Moss. “Wearing masks, especially indoors and in crowded outdoor settings, will help us contain this more transmissible variant.”

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The Covid-19 strain known as Delta is in at least 60 countries including the U.S. and likely to spread world-wide, Covid-19 Genomics UK Chair Dr. Sharon Peacock tells WSJ's Betsy McKay at the WSJ Tech Health event. (Video from 6/9/21) The Wall Street Journal Interactive Edition

Health officers across California have also continued urging unvaccinated residents to get their shots. The virus is surging in areas with the lowest vaccination rates, and unvaccinated people account for the vast majority of new cases and hospitalizations, according to state health officials. On Thursday, the University of California system announced it wouldn’t allow unvaccinated students back on campus this fall.

Peter Chin-Hong, an infectious-disease specialist and professor of medicine at the University of California San Francisco, said the push to mask up again was an “emotional reaction” to the rising case numbers, as health officials look for something they could do to slow the spread. He expected more counties would adopt mandates, not just recommendations, in the coming weeks, partly in response to big counties like Los Angeles doing so.

“We’re seeing the domino effect across the state,” Dr. Chin-Hong said.

The new recommendations and requirements, he said, reflected the possibility that infections among fully vaccinated people are becoming more common. Even though their cases are usually very mild, “the chance of transmitting to unvaccinated people is the justification for this mandate,” he said.

Dr. Chin-Hong questioned, however, how effective the new mask recommendations and mandates might be. Even in counties like Los Angeles, where the vaccination rates are lower than in the Bay Area, with 61% of residents ages 16 and over fully vaccinated, the high rates among seniors means a spike in hospitalizations and deaths like the area endured in the winter would be unlikely, even without new mask requirements, he said. During most of the pandemic, Americans aged 65 and older, nearly 80% of whom are now fully vaccinated, faced the highest mortality risk.

“Even in Los Angeles, the vaccination rate over 65 is very high,” he said. “That’s what informs deaths.”

Write to Ian Lovett at ian.lovett@wsj.com