More than two-thirds of Californians have antibodies to the COVID-19 virus, according to new data, offering evidence that we’re approaching so-called “herd immunity” — the pandemic’s endgame.
In just one month, the percentage of Golden State residents showing some immune protection has soared from 47% to 67%, and is likely higher now, according to an analysis presented by state epidemiologist Dr. Erica Pan this past week. While some of that protection is attributable to past infections, the recent jump between March and April suggests that the state’s expanded vaccination campaign deserves the most credit.
“That is a spectacular improvement,” said Dr. Steven Goodman, professor of epidemiology and population health at Stanford University. “It’s still less than we want. But if we have this big a jump in the next month, then we’re in great shape.”
But with variations across the state – 48% in the far northern region, 68.7% in the Bay Area and 76% along the southern border – there remain geographic differences in risk for those who are still unvaccinated.
It now appears that parts of the state are edging towards herd immunity with very low levels of viral transmission in some communities, according to UCSF epidemiologist Dr. George Rutherford.
“I think we’ll reach something that looks like herd immunity at some point in time — probably in the next couple of months,” he said, adding that the Bay Area could be among the first regions to reach that coveted goal.
There is optimism nationally, as well. If vaccination rates stay high and vaccines work against variants, infections could fall to low levels this summer, according to a recent report by the U.S. Centers for Disease Control and Prevention.
There has been recent hand-wringing about whether the United States will ever achieve complete herd immunity, the point at which so many people have antibodies that a virus stops spreading. But infectious-disease experts say it’s not an all-or-nothing goal. As populations gain immunity, infections slow — so geographic regions, if not the entire state or nation, become safe.
Infectious-disease experts say that the virus is unlikely to be eradicated. Instead, they predict that it will cause sporadic outbreaks but will then die out.
What percentage of a population must have COVID-19 antibodies to offer this widespread protection to the whole community?
We don’t yet know, said Dr. Art Reingold, professor of epidemiology at UC Berkeley. It depends on which model is used, with estimates ranging from 70% to 90%. The threshold can change and is determined by many factors beyond vaccination, such as population density and viral contagion.
“There’s no magic cutoff,” he said. “It’s really more of a continuum,” with vulnerability falling as antibodies levels rise, rather “than a ‘yes or no’ concept.”
There’s another unknown: A second arm of the immune system, which triggers so-called “cell mediated immunity,” may also be protecting us, Reingold added. That’s not measured in these antibody tests.
In the Bay Area, 68.7% of residents showed evidence of the antibodies, produced in response to a vaccine or infection, in tests conducted in April. That compares to 40.5% in tests conducted in March, according to the state data.
The state’s highest level of immunity – 76% — was along our southern border with Mexico, which suffered from many infections early in the pandemic. The lowest – 48.8% — was in the state’s northern expanse, in populations less touched by the virus and also more resistant to vaccination.
For its survey, the state used a test that measures so-called “neutralizing antibodies,” which are proteins that prevent infection by binding to the part of a virus that latches onto and enters a person’s cell. Antibodies are produced after infection and also after vaccination.
There are major statistical caveats. Because of the state’s sample sizes, these are fuzzy estimates; the true number may be higher or lower. For instance, antibody levels in the Bay Area could range from 60% to 78%. Because of this range, there may be less statewide variation than it seems.
In addition, the people sampled in a region may not have been representative of its entire population. For instance, they may have been younger or older, or were more willing or less willing to be vaccinated. The antibody testing data reported to the California Department of Public Health was collected from individuals at blood banks and doctors’ offices, among other sites.
A better approach, according to epidemiologist George Lemp, would be to conduct random testing at essential locations, such as post offices and grocery stores.
And even within one region, there could be pockets of infection. The regional number is just an average.
“We can’t just look at large regional numbers to know where our risk is,” said Goodman. In some Bay Area communities, as many as 90% of eligible people may be vaccinated, he said. In others, it may be only 40%.
This poses a challenge for state health officials considering whether to lift mask mandates, experts agreed. What is appropriate for one locale could be harmful to another.
“We’re a large state,” said Reingold. “You could have herd immunity in a small town in northern California and no herd immunity in Los Angeles… We’re not going to get herd immunity for the whole state, in the foreseeable future.”
In the pandemic’s endgame, vaccinating the last 20% of residents will prove harder than anyone thought it would be four months ago, according to a survey by the Brookings Institution. The pace of vaccination is slowing and those who remain undecided may be harder to persuade than those who have already made up their minds.
“The risk of getting infected is declining. But it is not zero,” Reingold said. “If you want to protect yourself, you shouldn’t rely on other people being immune. You should get vaccinated.”