California may not reach herd immunity for years. But vaccines will make the coronavirus more manageable FacebookTwitterEmail 1of5 Mission Street in S.F. attracts a crowd. Herd immunity occurs when enough people are immune so a virus can no longer spread.Scott Strazzante / The ChronicleShow MoreShow Less 2of5 A health worker helps direct people waiting to be vaccinated during a pop-up COVID-19 vaccination clinic in Gilroy.Stephen Lam / The ChronicleShow MoreShow Less 3of5 4of5 Patricia Lugo looks toward firefighter-paramedic William Nguyen of the Gilroy Fire Department as Nguyen prepares to administer a dose of Moderna COVID-19 vaccine at the Gilroy Senior Center.Stephen Lam / The ChronicleShow MoreShow Less