When Idaho had a rare measles outbreak a few months ago, health officials scrambled to keep it from spreading. In the end, 10 people, all in one family, were infected, all unvaccinated.

This time, the state was lucky, said the region’s medical director Dr. Perry Jansen. The family quickly quarantined and the children were already taught at home. The outbreak could have been worse if the kids were in public school, given the state’s low vaccination rates, he said.

In Idaho last year, parents opted out of state-required vaccines for 12% of kids entering kindergarten, the highest rate in the nation.

“We tend to forget that diseases like measles and polio used to kill people,” said Jansen, medical director of the Southwest District Health Department, which handled the outbreak in September.

All states require children to have certain routine vaccines to go to public school, and often private school and day care, to prevent outbreaks of once-common childhood diseases like measles, mumps, whooping cough, chickenpox and polio. All provide exemptions for children who have a medical reason for avoiding the shots. Most also offer waivers for religious beliefs. Fifteen allow a waiver for any personal belief.

Last school year, vaccination waivers among kindergartners hit an all-time high: 3% in total, according to a recent Centers for Disease Control and Prevention report. Waivers for religious or personal beliefs have been on the rise, driven by some states loosening laws, in others by vaccine misinformation and political rhetoric amplified during the COVID-19 pandemic.

In Idaho, “a parent only has to provide a signed statement,” to get a waiver, the state’s health department said. A change in state law before the 2018-19 school year made it easier to get waivers. The state’s exemption rate that year was 7.7%.

September’s measles outbreak started when a resident of Nampa, the state’s third largest city, returned home from a trip abroad. Measles is usually brought into the U.S. through travel since widespread vaccination has all but eliminated local spread of the disease.

It takes a very high level of vaccination — around 95% — to protect against the spread of measles and other diseases, experts say. During the pandemic, the national rate for vaccinations among kindergartners dropped to 93%.

Health experts say interventions on every level are needed to get more kids immunized: doctors talking to parents, social media campaigns, easier access to vaccines in some areas, enforcement by schools in others.