Will San Francisco really be the first U.S. city to reach herd immunity?
San Franciscos COVID-19 vaccination campaign is nearing the key benchmark of 70% of eligible residents fully vaccinated  which some experts say puts the city on track to be the first in the U.S. to reach herd immunity. The citys vaccine tracker showed that as of Wednesday, 79% of residents 12 and older had received at least one vaccine dose, and 69% were fully vaccinated. The definition of herd immunity varies, but experts generally say at least 70% of the population needs to be immune, either by vaccination or natural infection, for the virus to stop spreading. Some put the percentage as high as 80% to 90%. If San Francisco were to reach herd immunity, it would mean that coronavirus cases, hospitalizations and deaths could be pushed down to almost insignificant levels. If an unvaccinated person brought the virus to the city from elsewhere, it wouldnt lead to an uncontrollable outbreak. Bob Wachter, chair of UCSF's Department of Medicine, tweeted on Tuesday that with San Francisco reporting about 12 new cases per day and achieving such high vaccination rates, it may well be the first US city to reach herd immunity, citing a Guardian story about that possibility. In an email to The Chronicle on Wednesday, Wachter said that unless the entire world reaches herd immunity, coronavirus cases can still enter any community, so eradicating the virus wont be possible. But I do think the possibility of such a case leading to significant spread and a community-wide surge is getting close to zero in San Francisco and the Bay Area, he said. San Francisco is at a very high level of immunity (mostly from vaccination, with a small additional amount of immunity stemming from prior infection), which produces significant downward pressure on both new cases and on the propagation of cases that enter our community, he wrote. Whether we label this as herd immunity or recognize that the term is really shorthand for the much more complex idea that we are highly protected from COVID surges isnt all that relevant, clinically or scientifically.  Dr. Monica Gandhi, an infectious disease expert with UCSF, said Wednesday that San Franciscos high vaccination rate and persistently low case rates indicate the city could reach that coveted herd immunity status. I do believe we are on track to be the first city to achieve herd immunity, she said. This happened in Israel, which has now fully reopened after an 81% first vaccination rate and is still not seeing increased cases despite free mingling of vaccinated and unvaccinated. Our high rates of immunity means we are not susceptible to new infections even with travel here.  In a webinar Tuesday on San Franciscos approach to the states June 15 reopening, health officer Dr. Susan Philip indicated that vaccination rates in the city are so high that unvaccinated people are also fairly well protected in resuming daily in-person activities  which is a key element of herd immunity. Still, San Francisco and the Bay Area are not a bubble, and some experts say that herd immunity may never be truly reached. Philip said last month that San Francisco is not thinking about herd immunity in the sense of reaching a certain percentage that causes the virus to go away. Instead, she said, the focus is finding a different way to live with the virus.  John Swartzberg, an infectious disease expert at UC Berkeley, said he would prefer less focus on the idea of herd immunity because its a moving target, especially with the Delta variant that appears to be very contagious and is becoming more of a concern around the world. He said the herd immunity percentage can go up or down: If a community is locked down and human interaction is minimal, it decreases. When Californias economy reopens June 15, the target percentage could possibly increase. The way we should use this concept is to inform us of how were 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 on to, is a helpful measure to indicate if a community is nearing herd immunity. The goal is for the R-effective to drop well below 1. Californias coronavirus dashboard showed an R-effective of 0.81 for San Francisco as of Wednesday. Swartzberg said hes not interested in the herd immunity number, but rather how many people are vaccinated, how well the vaccines are working, how careful people are being, and most importantly, what are the number of new cases per 100,000 that were seeing, or what are the number of hospitalizations were seeing. If those numbers are going down, those are the numbers I care about.  When asked Wednesday about reaching vaccination milestones, the San Francisco Department of Public Health sent the following statement:  Although we are encouraged that San Francisco has very high vaccination rates, reaching a certain percentage of people immunized does not signify an end to COVID. There are some populations that continue to be susceptible to the virus, including communities that have lower vaccination rates as well as those people who are ineligible for vaccines, such as children below 12 years of age. Thus there is a need for ongoing vaccination efforts and closely tracking COVID cases.  The department said that rising vaccination rates will likely help the virus become a manageable threat that might continue to circulate in the United States for years to come, and still lead to hospitalizations and deaths but in decreased numbers. Kellie Hwang is a San Francisco Chronicle staff writer. Email: kellie.hwang@sfchronicle.com Twitter: @KellieHwang