Editor's note: This article was updated on July 28 to report that state officials have decided to follow the new CDC guidelines.
Nearly all vaccinated Californians should return to wearing masks indoors under new federal guidelines issued Tuesday for areas where COVID-19 is surging.
The new guidelines from the U.S. Centers for Disease Control and Prevention apply to regions with "high or substantial" transmission rates, which includes 45 of California's 58 counties and about 96% of its nearly 40 million people.
The guidelines would cover all of California's most populous counties. The counties, with lower COVID-19 rates, that are not included are: Monterey, San Luis Obispo, Santa Cruz, San Benito, Glenn, Tehama, Lassen, Modoc, Sierra, Alpine, Mono, Inyo and Tulare. About 1.7 million people live in those counties.
The announcement reverses an earlier CDC recommendation, issued in mid-May, that it was safe for vaccinated people to remove their masks in most settings.
All unvaccinated people, including children not yet eligible for COVID-19 vaccines, should continue to wear masks in all public indoor places under state and federal recommendations.
The CDC also recommended on Tuesday that vaccinated people in all areas of the country wear masks indoors if they are immunocompromised or have a higher risk of severe disease if infected, or if they live with someone who is. It also issued guidelines that everyone who attends, works at or visits a K-12 school wear masks regardless of vaccination status.
As in other states, California's COVID-19 cases have spiked in recent weeks as the more infectious delta variant has become dominant. Nearly 30% of eligible Californians remain unvaccinated along with children too young to be immunized.
Nationally, nearly two-thirds of U.S. counties are experiencing high or substantial transmission rates, according to CDC data.
State health officials on Tuesday reported more than 7,700 new COVID-19 cases and five new deaths, compared to about 700 new cases on June 15, the state's reopening day.
In response, California Gov. Gavin Newsom on Monday said health care workers and state employees must be vaccinated or undergo weekly testing and wear masks. The federal government is expected to follow suit on Thursday, CNN reported Tuesday.
On Wednesday, the state public health department updated its mask guidance to align with the new CDC recommendations, asking vaccinated Californians to mask up in indoor public spaces.
The new federal guidelines were prompted by evidence showing that the delta variant may be more likely than other variants to cause breakthrough cases in vaccinated people, and those people may still carry large quantities of the virus that can be passed to others, said CDC Director Dr. Rochelle Walensky. But she also noted that such breakthrough cases remain rare.
In California, about 21,000 breakthrough cases have been identified among nearly 21 million fully vaccinated people – just .1%, according to state health data.
Some public health experts were relieved by the CDC's reversal on Tuesday after criticizing its previous guidance, issued before the delta variant became predominant, as misguided.
"The CDC's decision in May to apply one size fits all mask guidance regardless of vaccines or prevalence was always a bad one for this exact reason: every community is different, and public health should be flexible enough to accommodate those distinctions," University of Saskatchewan virologist Angela Rasmussen tweeted Tuesday. "By oversimplifying complex and uncertain situations … we will not convince people to wear masks if they weren't already or persuade skeptics to get vaccinated."