Health officers from Alameda, Contra Costa, Marin, San Francisco, San Mateo, Santa Clara, and Sonoma counties and the city of Berkeley announced Monday that indoor masking is once again required in all public spaces, and recommended in private ones as well when you are gathered with people outside your household.

The new health order takes effect at midnight tonight, and you can now expect to see a lot more masks on in bars and restaurants. The estimated length of the new order was not announced.

The announcement came in a noon press conference, days after the U.S. Centers for Disease Control revealed new data that confirms that the Delta variant of COVID-19 is spreading among vaccinated people — and that vaccinated people, once infected, may be just as contagious as the unvaccinated, with viral loads reportedly 1,000 times that of the original variant. The CDC has not issued a national mask mandate, but it did recommend last week that areas of high transmission — which includes much of the Bay Area at present — should reinstitute masks in indoor spaces due to the high transmissibility of this variant, even for the vaccinated.

"We're facing a much more aggressive and contagious opponent right now," said Sonoma County Health Officer Dr. Sundari Mase during the press event. "This is not the same virus that we were combatting last year, or even a few months ago... and we're seeing an increase in the number of vaccinated people who are testing positive."

Dr. Naveena Bobba, Deputy Director of Public Health for San Francisco, added, "We know that indoor spaces are where most transmission occurs... If you are able to choose between an indoor and an outdoor space, we recommend that you choose outdoor activities. But some people have to work indoors... so we are urging people to mask indoors [to protect them]."

The health officers emphasized that vaccinated people are still all but guaranteed to have a mild or moderate case of COVID-19, even from the Delta variant, and the vast majority of recent hospitalizations have occurred among the unvaccinated and elderly. Also, being vaccinated will protect many from becoming infected at all.

Still, this is a wily and still not fully understood disease, and experts like Dr. Eric Feigl-Ding, a senior fellow at the Federation of American Scientists, have been sounding alarms that even mild cases can, in some instances, result in long-term cognitive impacts.

Today's new mask mandate has been expected for over a week, with infectious disease experts around the country saying that masks and vaccinations are our best defense against the ongoing surge in cases — not to mention the best defense against the rise of yet another, more pernicious variant.

Several Bay Area cities already reissued mask mandates of their own, including San Jose and Mountain View, and Los Angeles reinstituted its mandate two weeks ago. San Francisco's health officer, Dr. Susan Phillip, issued just a recommendation for indoor masking on July 16 along with several other regional health officers, prior to the broader understanding about transmissibility of the Delta variant among the vaccinated. That understanding came largely out of well-documented contact tracing and data from a July Fourth outbreak in Provincetown, Massachusetts — where a large number of gay men from the Bay Area were, bringing their infections home.

The hope is that this surge will drop off within weeks for the Bay Area region due to our high rate of vaccination, much as similar Delta surges have dropped off in the UK and the Netherlands.

As SF's Director of Public Health Dr. Grant Colfax has said of the variant, "It is COVID on steroids. In many ways, this is a different virus than the virus we were dealing with earlier this year."

In San Francisco, many residents have already begun masking up while grocery shopping again, and some people never put away their masks at all. Since June 15, bars and restaurants in the city have been bustling and often crowded with mostly unmasked patrons, and as of last week, the SF Bar Owners Alliance announced that proof of vaccination or a negative COVID test less than 48 hours old would be required for entry in hundreds of city bars.

With a new mandate about masking coming from county governments, it takes the onus off of individual businesses from creating their own mask policies — which could be bad for business with less cautious residents more than done with mask-wearing having just spent six weeks with few rules outside of public transit and Uber/Lyft cars.

Brahm Ahmadi, the owner of Community Foods Market in West Oakland, has already seen the rage among customers after the store reinstituted a mask mandate two weeks ago. As he tells the Chronicle, customers have verbally and physically assaulted his staff over the mask rule, including one customer who threw an entire bag of groceries at a cashier last week.

The coalition of Bay Area health officers said the mask mandate was necessary in order to avoid any further shutdowns of indoor businesses, especially as ICU beds begin filling up again. Dr. Chris Farnitano from Contra Costa County's health department said today that he's hopeful that no further restrictions will be necessary, and that hospitalizations will not spike as high as they did during the winter surge due to our high vaccination rates overall, and more vaccinations still occurring.

The CDC continues to stress that breakthrough infections, even with Delta, remain rare — and the Provincetown outbreak indicates one scenario, with thousands of strangers crammed into small indoor and outdoor spaces and partying for days, in which the Delta variant has more chances to overwhelm vaccinated people's immune systems.

Across California, the test-positivity rate for COVID-19 is now 6.7%, almost one percent higher than a week earlier. In San Francisco, where 70% of the population is fully vaccinated, the seven-day test-positivity rate was 5.3% as of July 25, and the seven-day average of daily new cases is up to 198 — nearly 20 times higher than the low of 10 seen in mid-June, before the state's big reopening.

Millennials appear to be driving a large portion of the new cases, due probably to their active social lives — and as the Chronicle reports via city data, residents aged 25 to 39 represent 50% of all new cases, and only 66% of city residents aged 25 to 34 are fully vaccinated, four percentage points below the city at large.

Previously: CDC Confirms That Viral Loads In Vaccinated People With Delta May Be Infectious, So Masks Are Necessary