States are heatedly debating whether to make it more difficult for students to avoid vaccinations for religious or philosophical reasons amid the worst measles outbreak in decades, but schoolchildren using such waivers are outnumbered in many states by those who give no excuse at all for lacking their shots.

A majority of unvaccinated or undervaccinated kindergartners in at least 10 states were allowed to enroll provisionally for the last school year, without any formal exemption, according to data reported to the Centers for Disease Control and Prevention. Only 27 states submitted information about the group, so the true size of the problem is unknown.

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Poor access to health care keeps some of those children from getting inoculated against some of the most preventable contagious diseases, but for others the reasons are more mundane.

"It really could just be, 'I didn't have time to go to the doctor,' or 'I just don't want to do this,'" said Melissa Arnold, CEO of the American Academy of Pediatrics' Ohio chapter. "From a public health standpoint, we really don't know."

Experts say it's likely that many or even most of those children ultimately get all their vaccinations, as state laws require, but no one knows for sure. It's neither tracked nor required to be.

That leaves officials with a maddening lack of information as vaccination rates inch downward and diseases like measles, once declared eradicated, reemerge.

The CDC has called on education officials to do more to ensure that those children get vaccinated, and state health and education departments routinely issue reminders. But for school officials, complying with state mandates that require children be vaccinated in order to attend class can sometimes require choosing between educating students and safeguarding public health.

"At the heart of our purpose is to have children in school; that's our role as school nurses," said Kate King, a board member at the Ohio Association of School Nursing. "We don't want to exclude them. So that's our dilemma."

All 50 states allow students to receive exemptions from vaccinations for medical reasons. But formal vaccine exemptions for religious or philosophical reasons have recently come under fire as the CDC has confirmed 880 measles cases in 24 states since January, the greatest number since 1994.

But children whose vaccinations are incomplete for other reasons can't be ignored.

Of the 27 states that reported data on that group for the 2017-2018 school year, Arkansas had the highest percentage of kindergarten students enrolled without complete vaccinations and without invoking a medical, religious or philosophical exemption, according to the CDC . In Ohio, that figure was 5.3 percent, the second highest. Georgia and Hawaii were lowest, at 0.2 percent.

Neither Ohio nor Arkansas has any measles cases yet this year, but health officials say the percentages of unvaccinated children are a worry. A 95 percent immunization rate is considered necessary to achieve group resistance to the spread of a contagious disease, officials said.

"If it gets here, it will be bad," King said of Ohio.

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In the 10 states where unvaccinated kindergartners lacking exemptions outnumbered unvaccinated kindergartners who invoked them, the figures were striking: Only about 15,000 children were using exemptions compared to almost 27,000 who weren't. Overall, the 27 states reported about 60,000 kindergartners who were unvaccinated without exemptions and about 70,000 who used them.

States provide anywhere from a few days to many months for students to get vaccinated, but officials in charge of the vaccination data for several different states said no system is in place to go back and check whether children ever get caught up.

Once a grace period expires, barring a student from attending school can be a tough call, risking the child's educational outcomes and, in some urban districts, their safety.

In Pennsylvania, officials recently shrank the state's eight-month grace period to just five days, said Cindy Findley, the state's acting deputy secretary for health promotion and disease prevention. The shorter window brings more focus and resources to the issue at the busy start of school.

"What we'd find is children would go through the entire school year and not be up-to-date with their vaccinations and basically carry on to the next age group," she said.

Other states, including Arkansas and Indiana, now require public reporting of kindergarten immunization rates by schools, and Colorado has made the information easier to access. Dr. Jennifer Dillaha, medical director for immunizations at the Arkansas Department of Health, said the idea is to draw attention to the issue and to provide parents with information that might affect their choice of school.

The CDC theorizes that "vaccine hesitancy" — fueled by a vocal anti-vaccination movement that contends some shots are unsafe despite overwhelming evidence to the contrary — has contributed to rising levels of unvaccinated schoolchildren in the U.S. But in Arkansas, Dillaha said, the issue is access.

Most Arkansas children are on Medicaid, health insurance for low-income residents, and 16 of 75 Arkansas counties have only the local health department to turn to for vaccinations, she said. That means no doctor's offices, clinics or corner pharmacies to make the procedure convenient.

"We have a weak immunization infrastructure," she said. "Consequently, because there are access issues, it varies from school to school how rigidly they enforce attendance requirements for vaccinations."

In Ohio, state data show the number of unvaccinated students remains high as students go through the school system, with 10 percent of seventh graders last year undervaccinated without invoking an exemption.

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Ohio's last measles outbreak occurred in 2014 Knox County and was traced to the local Amish community, where vaccination rates trail the general population's because their traditional lifestyle tends to eschew anything but the most vital medical care, said Pam Palm, a spokeswoman for the Knox County Health Department.

But even with that history, officials try to be flexible. Steve Larcomb, superintendent of the East Knox Local Schools in Knox County, recalled accommodating a parent who was new to the district, busy moving and awaiting a child's doctor's appointment five weeks in the future.

"We try not to draw too many lines in the sand and be too hard core, because we understand family situations," Larcomb said.