About 80% of adult Bay Area residents have detectable antibodies to the COVID-19 virus, up from about 70% a month ago, suggesting that the region is edging towards herd immunity — increased safety due to very low levels of viral transmission.

Statewide, immunity is not much lower, at 76.6%, according to an analysis presented by state epidemiologist Dr. Erica Pan on Wednesday. That, too, represents improvement.

“We’ve made huge strides on this,” Pan told the state’s Community Vaccine Advisory Committee this week. “Seeing this increase over time, with vaccination, is very encouraging.”

The state’s studies, known as serological surveys, involve testing the blood of healthy people — taken at blood banks or doctor’s offices — to determine whether they carry antibodies from prior infection or vaccination.

These findings are extrapolated to suggest what they mean for a region as a whole. And the data can help policymakers and medical experts gauge how vulnerable a community remains to the virus.

But the surveys are not random and do not reflect a community’s true socioeconomic, geographic, age and ethnic diversity. For instance, they do not include people under age 12, who are not yet eligible for vaccination.

With variations of antibody levels across the state, there remain stark geographic differences in protection.

The state’s highest level of immunity — 86.5% — is along our southern border, which suffered from major outbreaks early in the pandemic. The lowest — 54.4% — is in the state’s northern expanse, in rural communities less touched by the virus and also more resistant to vaccination.

With mask mandates rolled back, a significant number of people still remain susceptible. As the highly transmissible new Delta variant gains a foothold in the nation, these people face increasing risk of infection.

“The Delta variant first identified in India is currently the greatest threat in the U.S. to our attempt to eliminate COVID-19,” Dr. Anthony S. Fauci, the nation’s leading infectious disease expert, said at a White House briefing on Tuesday.

Here’s the good news: Studies have shown that the available vaccines work against all variants, including the Delta variant. The Pfizer and Moderna vaccines are 96% protective against hospitalization. The J&J vaccine has not been tested against the Delta variant, but the Astra Zeneca vaccine, which is similar, is 92% protective. It’s likely that prior infection also protects against the Delta variant, but that has not been studied.

“As worrisome as this Delta strain is with regard to its hyper-transmissibility, our vaccines work,” Centers for Disease Control and Prevention Director Dr. Rochelle Walensky said last week.

The state’s new analysis also provides other insights:

• Nearly all COVID-19 deaths in California now are in people who weren’t vaccinated.

Deaths among fully vaccinated people account for only 54 of the state’s roughly 36,606 deaths since Jan. 1. That’s about 0.15%. These so-called “breakthrough” infections after vaccination are more likely to occur if people are immunocompromised and do not mount a strong response to the vaccine.

• Rates of infection and hospitalization were 80% lower in Californians who were vaccinated than in those who were unvaccinated.

•  Young people are still behind on vaccination. Only 40% of people ages 12 to 17 are fully or partially protected, compared to 64% of those ages 18 to 49 and 78% of those ages 50 and older.

“The Delta variant remains a significant threat to the people under age 18,”  said UC San Francisco epidemiologist Dr. George Rutherford.

When will we reach so-called herd immunity, with widespread protection of the whole community? It’s hard to say for sure because it depends on a variety of factors, such as the biological characteristics of the circulating virus, Rutherford said.

The original Wuhan virus was estimated to have a “reproductive rate” of three — that is, an average of three people are infected by one infectious individual. When that strain was dominant, experts anticipated that we’d reach herd immunity if 67% of the population was protected.

But the Delta variant is about twice as transmissible as the original virus. If that becomes California’s strain, as expected, we won’t reach herd immunity until 84% to 86% of the population is protected.

If both strains are circulating, along with others, we may need 80% or higher protection, said Rutherford.

Herd immunity is also influenced by how often people come into contact with one another. Crowded communities need higher levels of protection to be safe. We could have herd immunity in Piedmont, for instance, but not in South Los Angeles.

Additionally, a snapshot of antibody levels represents only part of the picture. There’s another arm of the immune response, involving T cells, that also offers protection by reducing risk of serious illness or death. But T cells aren’t easily measured.

At least the Bay Area, which stretches from Monterey to Sonoma counties, is getting close, although Rutherford said he’s not sure we’re there yet.

The Bay Area’s recent jump in evidence of antibodies, up from 40.5% in March, “is important,” said Rutherford. “It’s really great.”