Bay Area immunity rates among lowest in state

More than a third of Californians who were recently tested for antibodies appear to have some immunity against the coronavirus — either from a past infection or from the vaccine, according to estimates from the California Department of Public Health.

About 38% of Californians had antibodies against the coronavirus, according to estimates based on the results of antibody tests of 79,780 people between Jan. 30 and Feb. 20. The figures were presented Tuesday during a virtual meeting hosted by the California Medical Association, and are slated to be posted this week, state epidemiologist Dr. Erica Pan said during the meeting.

The tests were a combination of antibody tests taken by individuals and reported by clinical labs, and routine screening of blood donors at blood banks. They were not a random sample. The California Department of Public Health said the 38% may not be representative of the general population but is likely a “reasonable approximation.”

The numbers offer a clue as to how close to herd immunity the state and various regions are. Antibodies are not exactly the same as immunity, but they are a good proxy because they are an important part of the immune system and can be measured more easily than other immune responses.

“It’s really helpful as we think about vaccination going forward, what our underlying antibody immunity is right now,” Pan said.

According to the estimates, the Bay Area has one of the lowest levels of antibody immunity out of all the regions in the state, 29%, most likely because it’s had relatively low virus levels. The Bay Area region includes the nine usual counties plus Monterey and Santa Cruz counties in the state estimates.

By contrast, Los Angeles and the San Joaquin Valley have the highest levels of antibody immunity, 45% and 44%, respectively — most likely because they have had much higher levels of virus.

“Not surprisingly, for the most part it really maps well to where we’ve seen the most cases and disease and where we now see more immunity,” Pan said.

People’s immune systems produce antibodies after being infected or as a result of getting vaccinated. It’s hard to distinguish how much of the statewide 38% estimate is from past infection and how much is from vaccines.

But given the antibody tests were done in late January through midFebruary, when just a small number of people — mostly health care workers —

were fully vaccinated, it most likely reflects natural immunity, said Dr. Monica Gandhi, an infectious disease physician at UCSF.

“That will be some vaccine effect but not huge, because we were still doing health care workers in January,” she said.

The state estimates also indicate that, as of February, antibody immunity was higher, around 50%, in the lowestincome areas of the state, compared to roughly 30% to 35% in the highestincome areas, as measured by the California Healthy Places Index. The index looks at economic stability, housing, education and other factors that influence income.

“It shows what we already know, which is that some regions were harder hit than others. Those are places with more essential workers,” Gandhi said. “It definitely shows the general trends of where places have gotten more infections. Antibodies are a good surrogate for that.”