In interviews, 27 parents who called themselves anti-vaccine and anti-mask voters described strikingly similar paths to their new views. They said they had experienced alarm about their children during pandemic quarantines. They pushed to reopen schools and craved normalcy. They became angry, blaming lawmakers for the disruption to their children’s lives.

Many congregated in Facebook groups that initially focused on advocating in-person schooling. Those groups soon latched onto other issues, such as anti-mask and anti-vaccine messaging. While some parents left the online groups when schools reopened, others took more extreme positions over time, burrowing into private anti-vaccine channels on messaging apps like WhatsApp and Telegram.

Eventually, some began questioning vaccines for measles and other diseases, where inoculations have long been proven effective. Activists who oppose all vaccines further enticed them by joining online parent groups and posting inaccurate medical studies and falsehoods.

“So many people, but especially young parents, have come to this cause in the last year,” said Janine Pera, 65, a longtime activist against all vaccines who attended the Orinda protest. “It’s been a huge gift to the movement.”