In San Francisco’s upscale Pacific Heights neighborhood, some 200 students from kindergarten to the eighth grade attend classes at the private San Francisco Waldorf School. On any given afternoon outside of the cheerful, modern white building, parents congregate to wait for their kids. Chit-chat includes the typical fodder like play dates and birthdays, or who was cast in the school play. But occasionally the topic of illness arises — which is where things can take a turn towards the atypical. You might, for instance, hear about “chicken pox parties,” where healthy kids come over to sick kids’ houses to catch the disease.

Of course, there’s a vaccine for chicken pox. It’s been available since 1995, and is part of every state’s recommended vaccine schedule, which the majority of US children receive. But SF Waldorf represents an unusual population: only 35 percent of incoming kindergarteners are up to date on all their vaccinations, one of the lowest rates in San Francisco for a school of its size and vastly lower than the national average of 95 percent. Which puts SF Waldorf firmly in the crosshairs of a national debate.