Will San Francisco actually be the primary U.S. metropolis to achieve herd immunity?
San Francisco’s COVID-19 vaccination campaign is nearing the top benchmark of 70% of eligible residents who are fully vaccinated – which, according to some experts, puts the city on the path to being the first in the US to achieve herd immunity.
The city’s vaccine tracker showed that by Wednesday, 79% of residents 12 and older had received at least one dose of vaccine and 69% had been fully vaccinated.
The definition of herd immunity varies, but experts generally say that at least 70% of the population must be immune, either through vaccination or natural infection, in order for the virus to stop spreading. Some put the percentage at 80 to 90%.
If San Francisco achieved herd immunity, coronavirus cases, hospitalizations, and deaths could be reduced to almost insignificant levels. If an unvaccinated person brought the virus into the city from elsewhere, it would not lead to an uncontrollable outbreak.
Bob Wachter, chairman of the UCSF medical division, tweeted Tuesday that with San Francisco reporting about 12 new cases daily and reaching such high vaccination rates, “could be the first US city to achieve herd immunity,” citing a Guardian story about this possibility.
San Francisco @ 12 new cases / day, in a city of ~ 900K. 79% of people> 12 years of age received at least one injection, 68% were fully vaccinated. Bottom Line: We may be the first U.S. city to achieve herd immunity, as @guardian described. @PCH_SF & George Rutherford @UCSF cited. https://t.co/ZFl2Y6GrDx
– Bob Wachter (@Bob_Wachter) June 9, 2021
In an email to The Chronicle on Wednesday, Wachter said coronavirus cases can still invade any community unless the whole world reaches herd immunity so that eradication of the virus is impossible.
“But I think the possibility that such a case will cause significant spread and community-wide rise is nil in San Francisco and the Bay Area,” he said.
“San Francisco has very high levels of immunity (mostly from vaccinations, with a little extra immunity from previous infection), which is putting significant downward pressure on both new cases and the spread of cases that enter our community.” he wrote.
“Whether we refer to this as ‘herd immunity’ or acknowledge that the term is really short for the much more complex idea that we are strongly protected from COVID surges is not that clinically or scientifically relevant.”
Dr. Infectious disease expert Monica Gandhi at UCSF said Wednesday that San Francisco’s high vaccination rate and persistently low case numbers suggest the city may achieve coveted herd immunity status.
“I think we are well on our way to being the first city to achieve herd immunity,” she said. “This happened in Israel, which has now fully reopened after an initial vaccination rate of 81% and, despite the free mixing of vaccinated and unvaccinated people, still has no increased cases. Due to our high immunity, we are not prone to new infections even when traveling here. “
In a webinar on Tuesday about San Francisco’s approach to reopening the state on June 15, health officer Dr. Susan Philip points out that vaccination rates in the city are so high that even unvaccinated people are fairly well protected when resuming daily personal activities – which is a key element of herd immunity.
Still, San Francisco and the Bay Area are not a bubble, and some experts say herd immunity may never actually be achieved. Philip said last month that San Francisco “doesn’t think about herd immunity” in the sense of reaching a certain percentage that will cause the virus to go away. Instead, the focus is on “finding another way to live with the virus”.
John Swartzberg, an infectious disease expert at UC Berkeley, said he would prefer to focus less on the idea of herd immunity as it is a “moving target”, especially with the Delta variant which is very contagious seems and gives increasing cause for concern the world.
He said the percentage of herd immunity can go up or down: when a community is locked down and human interaction is minimal, it decreases. If the California economy reopens on June 15, the target percentage could potentially rise.
“We should use this concept to find out how we are doing and how the virus is behaving,” he said.
Swartzberg said using the R-effective, which represents the average number of people an infected person is likely to pass the virus to, is a helpful measure for indicating whether a community is nearing herd immunity. The goal is for the R-rms value to drop well below 1.
The California coronavirus dashboard showed an R-Effect of 0.81 for San Francisco on Wednesday.
Swartzberg said he wasn’t interested in the number of herd immunity, but “how many people are vaccinated, how well the vaccines work, how cautious people are, and most importantly, how many new cases per 100,000 we have?” see, or how many hospitalizations we see. If these numbers go down, those are the numbers that matter to me. “
When asked Wednesday about the achievement of vaccination milestones, the San Francisco Department of Public Health sent the following statement:
“While we are encouraged that San Francisco has very high vaccination rates, reaching a certain percentage of people vaccinated doesn’t mean the end of COVID. There are some populations who continue to be susceptible to the virus, including communities with lower vaccination rates as well as people who are ineligible for vaccines, such as children under the age of 12. Therefore, ongoing vaccination efforts and close tracking of COVID cases are required. “
The department said rising vaccination rates are likely to help the virus become a “manageable threat that could continue circulating in the United States for years to come” and will still result in hospital admissions and deaths, albeit in fewer numbers.
Kellie Hwang is a contributor to the San Francisco Chronicle. Email: kellie.hwang@sfchronicle.com Twitter: @KellieHwang
