A school in Asheville, North Carolina, has been inundated with dozens of cases of the chickenpox following high rates of parents refusing to vaccinate their children, claiming religious exemption, according to multiple outlets.

Asheville Waldorf School teaches children from preschool to sixth grade. At the beginning of November, more than 28 children had been infected, and by this past Friday, that number had grown to 36, The Washington Post reports. It’s the worst outbreak in the state since a vaccine against chickenpox became available 20 years ago.

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The vaccine has been incredibly effective, as some 90 percent of children used to contract the disease. Beforehand, about 4 million cases of the virus would appear annually in the U.S., more than 10,000 of whom would be hospitalized and between 100 and 150 of whom would die.

According to the Buncombe County Health Department, which has been tracking the local spread of the chickenpox since October, exemptions from vaccination were a significant factor in the spread, CNN reports. Dr. Jennifer Mullendore, the county’s medical director, said last year’s Asheville Waldorf kindergarten class had the highest rate of religious exemptions in the county and one of the highest in the state.

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Local paper the Citizen-Times reported over the 2017 to 2018 school year, 19 out of 29 kindergarteners were exempt from at least one vaccine, and 110 out of 152 students in the whole school hadn’t received the chickenpox vaccine. Data has not yet been gathered for the current school year, Mullendore said.

In the outbreak’s wake, the school is instituting serious restrictions to prevent more cases, according to CNN. For example, anyone who has the disease must stay home, and classmates of kids found to be contagious have to stay home for 21 days. Parents must provide proof of immunity through blood work or a doctor’s statement to get around these rules.

“[The quarantine is] our last resort in terms of containing and stopping the spread of the illness so that we don’t see more cases and so nobody ends up with a complication,” Mullendore told the outlet, adding that no complications nor hospitalizations have been reported so far.

But future incidents are certainly possible.

“We can’t predict who is going to end up with complications,” she said. “There are case reports of previously healthy children ending up in hospital because of chickenpox — or dying because of chickenpox.”

According to the Centers for Disease Control and Prevention, the situation in North Carolina is part of a growing anti-vaccination movement. The rate of children under 2 who haven’t received any vaccinations has quadrupled since 2001.

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“The school follows immunization requirements put in place by the state board of education, but also recognizes that a parent’s decision to immunize their children happens before they enter school,” the school said in a statement to a local radio station.

The chickenpox, caused by the the varicella-zoster virus, are an itchy, blister-like rash. Other symptoms include tiredness and fever. The pox usually appear first on the stomach, back and face and then may spread over the entire body. It’s especially serious in babies and people with weakened immune systems. The best way to prevent chickenpox is to get the vaccine, the CDC says.

According to the Centers for Disease Control and Prevention, no scientific study has found a link between vaccines and autism.