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Hundreds of child care centers across Minnesota are not following state requirements that parents get their children vaccinated for the measles or provide written exemptions, according to state health data reviewed by the Star Tribune.

The situation provides fertile ground for the highly contagious measles virus to continue to spread as the outbreak, which has sickened 68, continues into its eighth week.

It also is hindering public health officials in their efforts to control the outbreak, because vaccination records at some centers are so poorly maintained that they can't quickly identify which children are most at risk where exposures have occurred.

"It has been fairly challenging to get information from the centers," said Kris Ehresmann, infectious disease director at the Minnesota Health Department.

Half the people sickened in the outbreak have caught measles at day care centers, and many passed it on to other unvaccinated family members.

State law requires all day care centers to enroll only children who have the full set of vaccinations appropriate for their age, except children whose parents provide notarized statements exempting them from the requirement for medical or personal reasons.

But in an annual accounting for 2015 provided by the centers to the Health Department, 223 of 1,400 centers across Minnesota reported that 5 percent or more of enrolled children who were supposed to get the measles vaccine did not get the shot and did not have an exemption.

Altogether, that represents about 1,800 children — nearly 600 more than the number who actually filed a personal exemption.

Overall, 96 percent of the 68,700 children age 2 or older who are enrolled in child care centers have received the first dose of the MMR vaccine, which protects against measles, mumps and rubella. The second dose is usually given when a child is in kindergarten.

However, only 42 percent of the 1,400 centers have perfect MMR vaccination records — leaving pockets of unvaccinated children throughout the state's day care system.

The Minnesota Department of Human Services (DHS) regulates child care centers, but it reviews immunization records only on a case-by-case basis as part of inspections.

"DHS does not currently follow up on the immunization rate data for child care centers," the agency said in a statement. "However, we are considering increased immunization record monitoring and follow-up."

Public health advocates say that Minnesota makes it too easy for parents to avoid getting their children immunized.

"There's no question that as pediatricians we would like our state to have stronger immunization laws," said Dr. Dawn Martin, a physician at Hennepin County Medical Center who works with other pediatricians on promoting vaccinations. "A personal belief exemption in Minnesota is relatively easy for a family to obtain."

Minnesota is one of 18 states that allows families to opt out based on personal beliefs; however, several states want to make it slightly more difficult than signing a piece of paper.

"More states are trying to tighten that by requiring that the parent get some education information from a health care provider," said Diane Peterson, associate director for the Immunization Action Coalition.

Bills that would add a counseling component have been introduced at the Legislature but have not gotten a hearing, Peterson and Martin noted.

Only three states — California, Mississippi and West Virginia — do not allow personal or religious exemptions, but they do allow for medical exemptions.

Public health officials advocate high rates of vaccination because, in addition to protecting those who get the shot, it also protects those who can't get immunized, either because they are too young or their immune systems are weakened by chronic illness or disease, such as children who are undergoing chemotherapy.

"That is the herd immunity," said Dr. Wilbert van Panhuis, an epidemiologist at the University of Pittsburgh Graduate School of Public Health. Measles is less likely to turn into an outbreak if at least 95 percent of the population has immunity protection.

"If the number drops below 95, the chance of measles infection is going up," he said. "Children who can't be vaccinated will be at an extreme risk."

That's one reason New Horizon Academy, a Plymouth-based child care provider with nearly 60 locations, changed its policy recently to only accept new clients who have had all appropriate vaccinations. Existing clients who had personal exemptions will still be allowed, but new clients without the shots will be turned away.

"We start taking infants as young as six weeks, and we just can't protect them if we are not sure that everybody else is vaccinated," said Kelly Ashton, a company vice president. Children below age 1 are too young to get the MMR shot.

"We did that because there was a measles scare before this one," said Ashton. The company began rethinking its policy last year when it required all staff to have immunizations.

Another large provider, KinderCare, has redoubled efforts to track kids and shots.

"The outbreak put an increased focus on making sure that our immunization records are up to date," said Colleen Moran, a company spokeswoman based in its Portland, Ore., headquarters. "We found that there [are] actually a lot more children immunized with MMR, but we didn't have the paperwork on hand."

The records review has prompted some parents to get the needed shots, Moran added.

Advocates say parents need to rethink their decisions to skip vaccinations — for the health of their children and the public.

"This is what was predicted. They knew that there were big pockets of unvaccinated kids," said Peterson. "Now we have unvaccinated kids that are getting ill and getting hospitalized, and it is a big concern."

Glenn Howatt • 612-673-7192