COVID-19’s delta variant is quickly spreading in California. Here’s what you need to know.

For the first time since early January, COVID-19 cases and hospitalizations are on the upswing again, largely driven by the spread of the more contagious delta variant.

Public health departments in seven Bay Area counties and one city issued an advisory Friday morning recommending — not requiring — that everyone, regardless of vaccination status, wears masks in public indoor places. Los Angeles County became the first California county to reinstitute an indoor mask mandate Thursday.

Here are answers to all of your questions about the uptick, what’s driving it and what comes next.

How has the spread of COVID increased in California?

Cases are ticking back up but are nowhere near the levels they hit near the end of the winter surge in February.

The statewide test positivity rate is 3.5%, compared with 1.7% on July 1 and 0.9% when the state fully reopened June 15.

The positivity rate is the percentage of all coronavirus tests performed that are positive. “A higher percent positive suggests higher transmission and that there are likely more people with coronavirus in the community who haven’t been tested yet,” Johns Hopkins University says.

State and local health officials have said that the vast majority of the new infections are occurring among unvaccinated people. In Los Angeles County specifically, health Director Barbara Ferrer estimated that 99% of new cases, hospitalizations and deaths are among those who are unvaccinated. While there are some breakthrough infections among fully vaccinated individuals, such infections remain rare and severe disease is even more rare.

Of the 10 California counties reporting the highest new daily case rates, only one — Sonoma County — has a vaccination rate over 50%.

What percentage of COVID cases in California are delta variant?

The delta variant, which was first identified in India and is more contagious than the original COVID-19 strain but does not appear to cause more severe illnesses, now accounts for about 58% of COVID-19 cases in the United States, according to the latest data from the CDC, which has analyzed cases up until July 3.

The CDC data indicates that in the region comprising California, Nevada and Arizona, the delta variant is 62% of cases. 

The California Department of Public Health is monitoring the spread of variants throughout California. The state has developed a variant tracking page that explains how, which and why variants are tracked. At the bottom of the web page, the state also provides information on known variants and what proportion of variants have changed over time. Visit the state’s variant tracking site at

Which California and Bay Area counties have tightened mask restrictions to combat the variant?

Alameda, Contra Costa, Marin, San Francisco, San Mateo, Santa Clara, Sonoma and the city of Berkeley issued advisories Friday morning recommending — not requiring — that everyone, regardless of vaccination status, wears masks in public indoor places.

Marin County Health Director Dr. Matt Willis explained in a phone call that a mask recommendation helps prevent spread because vaccinated people who are infected and asymptomatic can pass the virus onto unvaccinated people who can develop severe illness. 

“Breakthroughs are more common than initially thought for the delta variant,” Willis said. “The clinical trials weren’t testing ineffectivity. They were testing how well that person was protected.”

Mayor London Breed hinted in a press conference Thursday that new mask guidelines may be coming. 

“We are considering, basically, providing guidance on suggested mask-wearing in certain instances,” Breed said Thursday in a press conference with reporters. “We do ask that people who are not vaccinated, when they go indoors, that they wear masks and those that are vaccinated we don’t necessarily have a mask requirement further than that, but we are looking at a change to the policy, but not necessarily a mandate.”

Napa and Solano counties responded via email that they will continue to align with state guidelines and monitor cases and hospitalizations.

“Napa County will not be more restrictive than state guidance,” Napa County spokesperson Danielle Adams wrote. “Although Napa County has had more cases, hospitalizations are still low. We will continue to monitor cases and hospitalizations.”

“Solano Public Health will continue to monitor the situation and remove barriers to vaccination by focusing on under-resourced neighborhoods and partnering with community-based organizations,” Solano County said in an email. “Vaccinating as many people as possible, as soon as possible, is our best defense against COVID-19, the delta variant and the harm it can do to our communities.”

Outside the Bay Area, two counties, Yolo and Sacramento, are recommending that all individuals — even those who are fully vaccinated — wear masks indoors. Those are only recommendations for now.

Los Angeles County issued a new health order reimposing a mask mandate indoors. It is unclear how long that mandate will last.

Will Gov. Gavin Newsom reinstate the mask requirement statewide?

The state’s Department of Public Health did not directly address the question of whether a state mandate will be reinstated, emphasizing instead that “vaccines remain the best protection against COVID-19, including the highly infectious delta variant.”

“Unvaccinated Californians are not only at much higher risk of getting COVID-19 than those who are fully vaccinated, but they are also far more likely to suffer severe illness, hospitalization, and death,” the department said in a statement. “As we continue to see the real and aggressive impact of the delta variant in rising case rates, we cannot stress enough how critical it is for eligible individuals to get vaccinated.”
In addition, the department said it “support(s) the ability of local health jurisdictions to enact stricter local public health guidance that is tailored to the situation in their communities, as some counties have done.”

Should I wear a mask even if my county doesn’t require it?

The answer to this question depends on who you talk to: SFGATE reached out to two experts at UCSF who have closely worked on the COVID-19 pandemic and their opinions differed. 

Dr. Monica Gandhi, an infectious disease doctor at UCSF, said in an email to SFGATE that she doesn’t think a new mask requirement is necessary.

“I don’t because I am very convinced that the approach by our top ID doctor (Dr. Fauci) in the country and the CDC is taking is sound,” Dr. Gandhi wrote in an email.  “They are very clear that they do not intend to recommend masks for the vaccinated countrywide (White House task force briefing July 8 21:33) but that we should focus on vaccination efforts and outbreak management with surge testing, treatment and vaccination for places in the country with high hospitalization rates among the unvaccinated.”

UCSF infections disease expert Dr. Peter Chin-Hong, on the other hand, said via email that he thinks, with travelers coming into the Bay Area from other parts of the country and with different levels of circulating virus and vaccination rates, wearing a mask indoors wouldn’t hurt, particularly as cases rise locally.

“The point is that we want to protect the unvaccinated as there are increasing reports of vaccinated persons getting infected,” Chin-Hong wrote. “This is still a rare circumstance, and vaccinated persons will rarely get ill after infection, but nonetheless a moving target. It would still be a bummer (school, work missed) to even get infected as a vaccinated person and anxiety provoking so wearing that mask indoors is not a bad idea for an intervention that is cheap and doesn’t hurt.”

Where is the variant spreading in the Bay Area?

We asked all nine Bay Area counties this question. Five responded:

Solano County spokesperson Jose Caballero, wrote in an email: “Like surrounding counties, Solano County has seen an increase in cases after the 4th of July. Most of the cases are in younger groups that have lower vaccination rates. For more statistical information, check out the Solano County COVID-19 and Vaccine dashboard.”

Sonoma County spokesperson Matt Brown wrote: “We have detected 68 cases of the delta variant so far. That’s not to say that there aren’t more in the county, that’s just how many have been confirmed through genotyping at the state lab, which is a lengthy process. We recently acquired new equipment to do genotyping in-house, so I suspect we will be able to identify more variant cases.”

Adams of Napa County said via email: “As of Tuesday 7/13 (when the Testing Data page was updated but this number is still current), there were 11 known cases of the Delta variant in Napa County out of a cumulative total of 10,143 cases. It’s not really possible to say percentage of cases with the Delta variant because not all lab specimens are sequenced to determine variants. We also do not track or report ‘active’ versus ‘closed’ cases, so we don’t have a way to say how many current cases are due to Delta, either. 7 of the 11 people who have tested positive for the Delta variant had been fully vaccinated at the time of their positive test, which is about 64%.”

Contra Costa County’s email read: “Our case rate (currently at 5.7 per 100k) has doubled over the past few weeks. Over the two weeks, 76% of the COVID-19 test samples sequenced at our public health lab were Delta. Keep in mind, this is just a sampling but it’s safe to say we have seen delta steadily grow.” The county also noted that it doesn’t have data on the number of people who test positive for the variant and have been vaccinated. 

Willis of Marin County said over the phone, “We’re seeing surges in cases. We’ve seen a quadrupling in our cases rates in less than a month. We were at less than one new daily case per 100,000. We’re now at 4.5 cases per 100,000 residents.”

Willis added that one in three new cases are breakthrough infections where a fully vaccinated individual tests positive.

“We’re not seeing corresponding surges in hospitalizations or deaths,” Willis said. “While we are seeing an increase in the proportion of our cases that are breakthrough cases, the protection of the vaccine is clear in preventing severe disease. That’s what we’re most interested in in the first place.”

He added, “While the delta variant seems to be breaking through vaccines more than other variants, the vaccine does protect from severe illness. The question may become: What should we be measuring? If these are asymptomatic people who are vaccinated that are being diagnosed with COVID-19 what is the public health implication of that?”

How bad is spread of the delta variant in San Francisco?

In highly vaccinated San Francisco, where 76% of those 12 and over are fully vaccinated, daily cases increased fourfold over the week ending July 7, for which there is full data, the San Francisco Department of Public Health said. Cases went from a low of 9.9 cases per day on June 19 to 42 new cases per day on July 7.

“Forward looking data through July 12 indicates that new cases will increase to at least 73 cases/day, a seven-fold increase since June 19,” officials said. 

UCSF Dr. Bob Wachter has been tracking San Francisco COVID numbers throughout the pandemic and said on Twitter on Thursday that the S.F. numbers are still “fairly low” and “cause for caution, not panic.”

“But this kind of uptick in SF (U.S.’s vaccination leader) shows that Delta is very real – the places w/ much lower vax rates may well get clobbered,” Wachter wrote. “Alas, doesn’t seem like there are many persuadables left.”

The city did not indicate how many of those cases were in unvaccinated individuals, but Gandhi said in an email that 99% of those who are in the hospital with COVID-19 across the county are unvaccinated (including the 19 in San Francisco).

“The most important thing to know about delta is that unvaccinated are susceptible,” Gandhi said.

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