At the heart of this fight is a complicated truth: California’s new student vaccination requirements haven’t yet begun, but the state already has a remarkably low number of outbreaks at schools.

Of the 2,321 nationwide school closures since August because of Covid-19, about 1 percent have been in California — even though the state accounts for 12 percent of the nation’s K-12 students, according to data from Burbio, a technology company that monitors outbreaks.

So some parents may be wondering: If masking, testing and other prevention strategies are working so well, why is the state adding an immunization requirement?…

“The science works, if you are very, very persistent and purposeful about implementing it,” Darling-Hammond told me. “I think there’s a human tendency to say — as soon as things look good — ‘OK, we can take our foot off the gas.’ We can’t.”