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California is optimistic that the worst of the COVID-19 pandemic is in the rearview mirror. But there are a number of things that could still go wrong and prompt a fourth wave, experts warn.

Dr. George Rutherford, an epidemiologist and infectious diseases expert at UC San Francisco, estimated that roughly 67% of a population needs immunity to COVID-19 before herd immunity can be established, meaning the spread of disease between people is unlikely.

It’s not a hard-and-fast percentage, Rutherford said; the threshold given by the U.S. government’s top infectious diseases expert, Dr. Anthony Fauci, is around 70% to 85%.

Nationally, about 40% of people are believed to have immunity to COVID-19, Rutherford said, either from vaccination or because they’ve been previously infected and have survived. About 26% to 39% of California’s population is probably immune — a figure lower than the national average “because we haven’t had so much infection and, as a result, have not had as much naturally acquired immunity,” Rutherford said last week at a campus forum.

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