Bay Area is close to herd immunity. But we may never get there - San Francisco Chronicle

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The Bay Area has among the best COVID-19 vaccination rates in the United States and several counties are approaching the threshold of herd immunity, where the virus eventually dies out because it can no longer find people to infect.

If the region was in a bubble, that could eliminate nearly all coronavirus infections. But with vaccine uptake already waning across California and the U.S. — and vaccines still unavailable to large swaths of the world — the level of global immunity required to squash the pandemic is almost certainly unattainable, at least for a long time to come, infectious disease experts say.

Herd immunity, once widely assumed the endpoint of the pandemic, is now mostly off the table. Last month, Gov. Gavin Newsom called the idea in California “illusory.” Even in the Bay Area, which probably has a better chance of reaching herd immunity thresholds than most other parts of the country, public health officials have abandoned that goal.

Instead, they’re focused on something simpler: getting as many people vaccinated as possible.

“We are not thinking about herd immunity,” said Dr. Susan Philip, San Francisco’s health officer. “That’s true for public health officials and scientists around the country, in the sense that people originally talked about it — which was once we hit X percent of people vaccinated, the concern about the coronavirus goes away.

“Now we see that’s not going to happen,” Philip said. “So we’re going to find a different way to live with the virus.”

It’s a subtle but important shift in the dynamics of the pandemic, and a recognition that although the coronavirus won’t be eradicated, it can become a much more tolerable companion.

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Herd immunity occurs when so many people are immune to a virus that it is no longer able to spread and sustain itself. Scientists don’t know the exact threshold that’s needed for the coronavirus. Infectious disease experts generally say at least 70% of the population needs to be immune — either by vaccination or previous infection — but it could be much higher.

But even if communities fall short of herd immunity, life can largely go back to normal if enough people are vaccinated. That should keep cases at a low level and the worst consequences of the pandemic — serious illness and death — at bay.

Reaching herd immunity levels of vaccination in a place like San Francisco would drive down coronavirus cases, hospitalizations and deaths to near-inconsequential levels. Outbreaks would be uncommon and containable. If a tourist, a commuting worker or an unvaccinated resident coming home from an overseas trip brought the coronavirus into the community, it wouldn’t spark uncontrollable spread of the disease.

“If the country or the rest of the world is continuing to have spread of the virus, San Francisco can never have zero virus,” said Dr. Steven Goodman, a Stanford epidemiologist. “So the question is, can we just keep it at a lower ebb, where the community for the most part is somewhat resistant? If the immunity we have can simply prevent serious disease and death, our relationship with this disease changes.”

San Francisco already is remarkably close to that level of immunity. The city’s high vaccination rates — 74% of people 16 and older, and 65% of the overall population, have gotten at least one shot. That has helped bring cases down enough to resume many normal activities, like reopening indoor bars and expanding indoor capacity at restaurants, movie theaters and gyms.

If San Francisco and the rest of the Bay Area can achieve and maintain something close to herd immunity, the pandemic may be mostly over in the region even as it carries on in other parts of the state, nation and world.

“We’re too limited by ideas and politics and demographics to get to herd immunity” nationally, said Dr. Jay Levy, an infectious disease expert at UCSF. “But it’s a wonderful milestone for San Francisco, for various populations.”

Herd immunity is a fluid goal, dependent on not just vaccinations but other variables, including variants that could render vaccines less effective. The fact that young children won’t be vaccinated for months means herd immunity is essentially impossible this year. Plus, no one yet knows how long immunity lasts, which means it’s not clear what it will take to maintain herd immunity.

Pedestrians wearing face masks cross the street in San Francisco on Friday, February 12, 2021. The CDC recently said wearing double masks can help protect against the spread of COVID-19.Nick Otto/Special to The Chronicle

Vaccination rates vary widely across the Bay Area, but in six of the nine counties, at least 70% of people 16 and older have gotten at least one shot. And most counties have surpassed or are approaching 60% of their entire population receiving at least one shot, even taking into account children under 16 who can’t be vaccinated, which suggests that within the next four weeks, roughly that percentage may be fully vaccinated.

The expected authorization of the Pfizer vaccine for 12- to 15-year-olds later this month could push vaccination rates higher.

Every Bay Area county except Solano has better vaccination rates than the state and national averages. For California, about 60% of people 16 and older are at least partially vaccinated. Nationally, the total for people 18 and older is 57%.

None of those numbers includes a critical component of herd immunity: people who have already been infected. Experts believe that 10% to 20% of people have been infected in most California counties, which means that some places with high vaccination rates may already be at or very close to herd immunity levels.

“If you look at the cases in California, a little over 3.6 million out of 39.5 million population, that’s about 9% that have been reported with COVID,” said Dr. George Lemp, an infectious disease expert who is retired from the University of California. Including unreported cases, the total could be closer to 20%, he said, “and that’s a conservative estimate. We’re closer to herd immunity than we think.”

San Quentin State Prison is one example of a population that has reached herd immunity largely because of infection. After a coronavirus outbreak last summer infected more than 2,000 inmates, or nearly 90% of the prison population, cases dropped abruptly. Since August, only about a dozen cases have been reported among inmates.

Coronavirus Pandemic

Some nursing homes may also be benefiting from herd immunity, thanks to vaccinations. In Sonoma County, senior residential care facilities began to reach herd immunity in mid-February, health officials said. There have been no new cases among care home residents in the county since March 5, and no resident deaths since mid-February. About 95% of residents and 85% of staff at these facilities are fully vaccinated.

“We have seen a drastic drop in cases and resulting hospitalizations and deaths after vaccination,” said Sonoma County epidemiologist Kate Pack. “These are very promising results we’ve seen in this population. And these results are beginning to be mirrored in our general population.”

Counties, of course, are far more open to outsiders than prisons or nursing homes. But herd immunity should play out similarly.

“You’re going to have certain pockets of places where people come and go. So you’re going to always have some importations,” said Dr. George Rutherford, a UCSF infectious disease expert. “You’re just not going to have transmission. You’ll have a case here or there, but not sustained.”

One important caveat with herd immunity is that reaching that threshold in a county doesn’t mean every neighborhood is equally protected. That’s another argument for vaccine equity, public health experts say. Overall high rates of immunity will provide a protective umbrella over an entire county, but if a specific neighborhood has very low vaccination rates, it will remain vulnerable to outbreaks if the virus finds its way in.

“The high, high, high degree of vaccination in California means we’re unlikely to see the surges we had before. But will we have pockets where it starts to be transmission again? Yes. And we have to pay attention to that,” said Dr. Kirsten Bibbins-Domingo, vice dean for population health and health equity at UCSF. “There’s a lot of unevenness in how we vaccinate.”

Erin Allday and Catherine Ho are San Francisco Chronicle staff writers. Email: eallday@sfchronicle.com cho@sfchronicle.com Twitter: @erinallday, @Cat_Ho

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