Most Minnesota students go to schools with high vaccination rates, but a few small districts have clusters of unvaccinated children.

State law requires students to be vaccinated but allows parents to opt out of the requirement for medical or philosophical objections.

Overall, fewer than 3 percent of Minnesota kindergartners opted out of all vaccines last year, but some school districts have no unvaccinated students while one charter school had 20 percent of its small class opt out last year.

That matters for disease transmission. A lone unvaccinated person can catch a disease, but several unvaccinated people together multiplies how many people can be exposed.

“You have pockets of susceptibility in certain localities where students aren’t vaccinated,” said Nicole Basta, an assistant professor of epidemiology and community health at the University of Minnesota’s School of Public Health. “That could potentially lead to an outbreak if a disease were introduced to that area.”

Minnesota’s largest school districts have low rates of vaccine opt-outs. Just 1 percent of St. Paul Public Schools kindergartners were unvaccinated, with Minneapolis Public Schools barely higher at 1.1 percent.

But 16 school districts in Minnesota had kindergarten classes with at least 10 percent of their students opting out of vaccines last year. An additional 32 had from 5 percent to 10 percent opting out.

Those districts with high opt-out rates tend to be small, with fewer than 100 kindergartners each. They also tend to be private or charter schools; only three of the 16 districts are traditional public schools.

The small size does mean vaccination rates can be erratic. The highest opt-out rate last year was the Prairie Creek Community School near Northfield, a school devoted to “hands-on learning” and “the principles of progressive education.” Prairie Creek had 20 percent of its kindergartners opting out of vaccines — but in a class of 30 children, that meant six were unvaccinated.

A 20 percent rate in the St. Paul district, by comparison, would mean about 825 unvaccinated kindergartners.

Prairie Creek’s director, Simon Tyler, said his school’s high opt-out rate last year appears to be an anomaly. This school year, all 30 of its kindergartners have received their shots, he said.

“We tend to have big fluctuations,” Tyler said.

He said his school has never had a major outbreak of a preventable disease, such as measles or pertussis. Nor has there been a lot of conversation about the high rate of unvaccinated children in last year’s kindergarten class.

In general, epidemiologists aim for 95 percent of a population to be immunized, Basta said. That helps protect unvaccinated people — including those who are unable to be vaccinated due to allergies or suppressed immune systems — by making it harder for a disease to find new people to infect. It’s a concept called “herd immunity.”

Anyone who’s not immunized against a disease can be in danger, but that danger is heightened when multiple unvaccinated people are together, Basta said.

Other states have similar patterns, with most schools having high vaccination rates but a handful of small schools with considerably more opt-outs.

In Washington state, for example — a state with an opt-out rate more than twice that of Minnesota — most districts had between 0 percent and 8 percent of students opting out. But there are 40 districts, all small, with rates between 10 percent and 36 percent opting out.

Katie Wolt, a health educator with Washington’s Office of Immunization and Child Profiles, said communities with low child-vaccination rates haven’t been the cause of all the state’s recent disease outbreaks. Some, for example, have been connected to immigrant communities with frequent travel to and from parts of the world where diseases such as measles are still common.

But some have been based in “communities that choose not to vaccinate,” she said, whether that reason has to do with philosophy or religious beliefs. Basta said studies in states such as California also have shown “clustering of parents deciding to opt-out their children,” instead of unvaccinated children being spread evenly across the population.

Ben Christianson, an epidemiologist with the Minnesota Department of Health, said the department likes to see as many people immunized as possible. But the department doesn’t yet know what leads some schools to have clusters of opt-outs and others to have high vaccination rates.

“It’s something we’re looking in to, but we’re still in the infancy of exploring what may be going on in these communities,” Christianson said. “I think there’s different reasons in all the different communities that have the higher rates.”