While California's city of San Francisco is still recording a small number of Covid-19 cases, they don’t appear to be gaining enough of a foothold in the population to trigger wider outbreaks. “This is what herd immunity looks like. You’re going to have single cases, but they’re not going to propagate out,” said Dr. George Rutherford, professor of epidemiology at University of California San Francisco. Nearly 80 percent of San Francisco residents eligible to receive the Covid-19 vaccine have received at least one shot and 68% are fully vaccinated. Given the number of people who have gained immunity through having had the coronavirus plus the number of vaccinations, some experts say the city already has enough residents with antibodies to have achieved herd immunity.