San Francisco is still recording a small number of coronavirus cases, about 13.7 per day, said Dr George Rutherford, professor of epidemiology at University of California, San Francisco, but they don’t appear to be gaining enough of a foothold in the population to trigger wider outbreaks.
“That is what herd immunity looks like,” Rutherford said. “You’re going to have single cases, but they’re not going to propagate out.”
Health officials still lack agreement on exactly what percentage of the population must be immune to Covid-19 to achieve the much-vaunted status of “herd immunity”, when so many people have antibodies against the virus that it can no longer spread widely through the community.
Throughout the pandemic, the vaccination goals to hit herd immunity have been a moving target, a briefing from Yale School of Medicine notes, with experts initially estimating that the virus would have difficulty spreading if 60 to 70% of the population was immune. With several variants in circulation, some of which are more contagious, the estimates have been climbing. Many experts now estimate that herd immunity will be achieved when 80 to 90% of residents have been vaccinated.
San Francisco has been close to that target. Nearly 80% of San Francisco residents eligible to receive the coronavirus vaccine have received at least one shot, according to data from the city’s health department, and 68% are fully vaccinated. Among the full population, Asians and Pacific Islanders have even higher vaccination rates than the city average, the stats show, while the Black population lags by about 16 percentage points.