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San Francisco is nearing herd immunity, some experts believe, a milestone in California’s efforts to gain control of the COVID-19 pandemic.

San Francisco has one of California’s highest rates of vaccination, with 72% of residents having received at least one dose. Only one other county in California — Marin, just north of San Francisco — has a higher rate of vaccination, with 75% of residents there at least partially vaccinated. Both San Francisco and Marin County’s rates are significantly higher than the statewide vaccination rate of 56%.

Herd immunity, also known as community immunity, is a a form of indirectly protecting people without immunity to a disease from an infection. It occurs when a significant percentage of the overall population is immune either through vaccines or from surviving a previous infection.

There is no definitive percentage at which herd immunity to COVID-19 is achieved. It can only be tested when essentially all restrictions are loosened, and officials observe whether disease transmission increases.

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