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Herd immunity in the U.S.? San Francisco offers reasons for optimism

The city kept case numbers low, and it has one of the highest vaccination rates of any major U.S. city. Now comes the real test.
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SAN FRANCISCO — Adam Bergeron is looking forward to reopening the Balboa Theater, the independent movie theater in San Francisco he owns and operates. 

He's watched as other movie theaters around the U.S. welcomed audiences back ahead of his. But San Francisco has been slower to reopen than other cities. Now, the time feels right.

"San Francisco’s been a model for doing Covid the right way, if there is such a thing," Bergeron said.

"At this point, everyone I know is completely vaccinated. The cases are going down. And we just chose a time that seemed like it was going to be the right time," he added. He’s planning to reopen May 14 with a "Godzilla" marathon.

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San Francisco and its suburbs have been cautious, maintaining various restrictions while other parts of the country reopened businesses and eased mask mandates. Meanwhile, its vaccination rate is among the highest of any major U.S. city, with two-thirds of all adults having received at least one dose. 

And as parts of the city open up — some San Francisco bars have waitlists to get a table again — experts are offering a cautious optimism. The city may be seeing signs of herd immunity.

"This is our moment to put the pandemic behind us," said Dr. Grant Colfax, San Francisco's health director. "It's clear that the vaccines are our way out of this."

Herd immunity is about transmission. If enough people can't catch and spread a virus, particularly in a community that already has few cases, the virus struggles to find new hosts. Eventually, infections would naturally taper off. It’s a simple concept, yet it can be elusive and difficult to define — particularly at the national level. 

Locally, though, it can be clearer. Over the past seven days, San Francisco, home to more than 870,000 people, recorded an average of only 26 new Covid-19 cases per day. Two-thirds of all adults in San Francisco and almost 60 percent of the greater metro area of 4.7 million have been vaccinated with at least one dose — one of the highest rates in the U.S. Its positive test rate sits at 1.2 percent.

Throughout San Francisco, there are signs of residents beginning to relax their emergency precautions, at least slightly, and enjoy a reward from vaccinations. Some people are going maskless during walks outdoors — a rare sight until recently — while small gatherings like running and cycling clubs have resumed and in-studio yoga and other fitness classes have restarted. Prompting a burst of municipal applause, the main branch of the city's library reopened Monday for browsing, and the city could enter California's "yellow tier" — the least-restrictive pandemic tier — this week.

And there are signs that the city is preparing for a long-term reopening. Kevin Carroll, president and CEO of the Hotel Council of San Francisco, a trade group for the city’s hotel industry, said a majority of San Francisco’s 34,000 hotel rooms are expected to be open by the end of May. More downtown offices, including those of major tech companies, are in the process of coming back. Salesforce, the software company that occupies San Francisco’s tallest skyscraper, is targeting a reopening of its headquarters this month, spokesperson Annie Vincent said. 

It's estimated that 70 percent to 85 percent of a city’s population will need to be immune to reach herd immunity, but that number can be a moving target. A better indication is if a city’s number of cases and hospitalizations fall to a low level and stay that way, even with few restrictions in place, said Dr. Monica Gandhi, an infectious disease physician and professor of medicine at the University of California, San Francisco.

San Francisco had the advantage of ramping up its vaccination campaign while cases, hospitalizations and daily deaths were already relatively low, which most likely accounts for much of the city’s success so far. This past winter, when much of the country was battling a devastating surge, San Francisco’s cases peaked on Jan. 4 with 560 reported infections. By comparison, Los Angeles recorded its peak on Dec. 26, with more than 29,000 new cases.

Vaccines can help bring those numbers down, but how quickly it happens depends largely on the situation on the ground. In places where cases are increasing, scientists have observed an inflection point with vaccinations, after which cases, hospitalizations and deaths start to fall dramatically. 

“It appeared to be at the 40 to 50 percent first-dose rate,” Gandhi said. “After that, things started plummeting.”

There are currently 15 people hospitalized for Covid-19 in San Francisco, a figure that roughly works out to be less than 2 per 100,000 people. While there’s no magic number, Gandhi said it’s these types of low hospitalization rates that public health officials monitor to ensure a city is on the right track.

In March 2020, San Francisco was the first city in the nation to issue a mandatory shelter-in-place order in response to the pandemic. And the city has maintained other stringent mitigation procedures that Colfax, San Francisco’s health director, said has helped the city “beat back three surges.”

Colfax also credited the widespread embrace of the city's public health interventions for that progress. 

"This culture and support for public health, and infrastructure for public health, is deeply embedded in San Francisco, largely due to our response to the HIV/AIDS epidemic," he said. "That's ingrained in our cultural DNA."

But the real test will likely come this week when the city begins to lift some of its restrictions.

"The final test when you achieve herd immunity is not being in lockdown and not having mitigation procedures," Gandhi said. "The way we could tell that we got to herd immunity with measles is that people were out and about and mingling and people weren't getting sick and kids weren't being hospitalized with severe measles."

Still, herd immunity is not a fixed target, and the threshold can vary depending on a number of factors, including population dynamics. Achieving herd immunity also doesn't automatically guarantee the coronavirus will simply fade away. Even though more than 90 percent of the U.S. population has received a measles vaccine, for example, there can still be outbreaks.

As such, herd immunity should not be thought of as the end goal, said Dr. Julie Parsonnet, a professor of epidemiology and population health at Stanford University.

"It's not something that is permanent, and just because we hit herd immunity doesn't mean there aren't going to be cases," Parsonnet said. "Herd immunity is a good construct for modeling but not for life."

There's also a risk that a variant of the virus could emerge that escapes the protection of vaccines. There is no evidence to suggest that has happened so far, but if it did, it could jeopardize the protection that communities have built up.

And while pockets of the country, like San Francisco, may have already reached a level of immunity that allows most major restrictions to be lifted, it could take a long time for the entire country to get there — if it ever does. But that doesn't mean lockdowns will linger for years or that life can't carry on.

"If we get to the point where the coronavirus doesn't make people very sick from it, we'll be in good shape," Parsonnet said. "If we don't get to herd immunity as a nation, we'll still be protecting vulnerable people with vaccines and hopefully not having hospitalizations."

While San Francisco may be the first major U.S. city that appears to have wrested control of the pandemic, others are likely not far behind. 

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More than 46 percent of people in New York City have received at least one vaccine dose, and cases, hospitalizations and deaths are all steadily declining. Los Angeles County, which less than five months ago was considered the epicenter of the pandemic in the U.S., recorded no new Covid-19 deaths on Sunday and Monday. Fifty-four percent of residents there have received at least one vaccine dose, and it’s among several counties in California poised to roll back restrictions this week. 

Elsewhere in the country, smaller cities such as Albuquerque, New Mexico; Portland, Maine; and San Diego are seeing similarly encouraging progress.

"It’s profoundly hopeful," Gandhi said. "The vaccines have made it so that we’re in an entirely new world."

Denise Chow reported from New York City, and David Ingram from San Francisco.