Last year, the U.S. recorded the most measles cases since the disease was declared "eradicated" in 2000.

It's a problem striking close to home as the Pennsylvania Department of Health finds one in five kindergartners may have entered school without being vaccinated.

In response, state officials want to adopt tighter rules mandating that children have all their shots by the end of the first week of school.

But those statistics -- and efforts to enforce the rules -- have been hampered by unreliable data.

Helen Kelly, a nurse at McKinley Elementary School in North Philadelphia, has a theory about that. Vaccination records, she said, often are lost in the shuffle during the first months of classes.

"A lot of our parents think, 'Oh, I did all that work for preschool ... I don't have to do it for kindergarten,'" said Kelly.

Kelly collects records for the state Department of Health's survey. Last year, that survey found only 73 percent of McKinley's kindergartners were up to date on the measles, mumps and rubella vaccine.

Meanwhile, the official citywide kindergarten measles vaccination rate jumped from 87 percent to 94 percent over the past two school years.

So which numbers are right? Is this a paperwork issue? Or does it reflect actual vaccine rates?

"What that says to me is that our ability to capture immunization rates may lead to some imprecision in our estimates. And I think really that reflects the challenge of measuring immunization rates, especially in schools," said Dr. Kristen Feemster, director of research for the Vaccine Education Center at the Children's Hospital of Philadelphia.

And getting reliable numbers is vital, she said.

"This is how we enforce our mandatory immunization requirements. So to have an effective policy, you need to have a way to effectively implement it," she said.

Because the Philadelphia School District is so strapped financially, many schools are sharing nurses.

So vaccine records are not a priority, Kelly said.

"I know there are a lot of nurses that just don't have it ready," she said. "They have so many other things going on in their building, that this takes a backseat sometimes."

Pennsylvania lags U.S.

Using the Pennsylvania Health Department survey, the Centers for Disease Control and Prevention ranked the state as having one of the lowest measles vaccination rates in the country.

Following reports from NBC 10 and Public Source that uncovered data inconsistencies statewide, Department of Health Secretary Dr. Karen Murphy pledged updates to vaccine education and verification. The survey's deadline was also moved from October to March.

Recently, Murphy called for giving kids who haven't received all their shots by the beginning of the school year just five days to get up to date, a proposal that will likely spur lots of debate.

Kelly said having more time to track and collect data will improve numbers; all of her first-graders at McKinley Elementary, for instance, are up to date. But it won't improve how she gathers the information.

"I can't keep sending a letter every week. I'll wait a month, and I'll make phone calls or try to meet the child's parent when they drop them off in kindergarten," she said.

The survey is now on track to be done by March. Dr. Paul Offit, director of vaccine education at CHOP, said this grace period helps meet the deadline. But, immunizations are supposed to happen before school starts.

"You want to give these vaccines before children are likely to be exposed. Otherwise, they'll get sick," he said.

Murphy is not only calling for kids to get all their shots during the first week of school, she supports shortening the reporting window next year so kids spend less time unprotected.

But even if those rules are adopted, school nurses across Pennsylvania face budget strains. Budget cuts mean more and more Philadelphia schools share nurses and are unable to track and enforce compliance in a timely manner.

Yet, Philadelphia has a tool other Pennsylvania cities don't — a not-so-secret weapon. All city doctor's offices are required to report childhood vaccinations to a database called KIDS IIS.

With better data from electronic records, KIDS' numbers are more consistent and don't rely on parents to fill out forms. It reports the minimum measles vaccination rate for Philly's 6-year-olds is 91 percent.

Preventing an outbreak

Offit emphasizes the recommended vaccination rate is in the high 90s. Then, the whole student population's protection, called herd immunity, can shield kids who cannot be vaccinated for medical reasons.

"Measles is starting to come back. That virus is so contagious that if we have immunization rates in the high 80s, low 90 percent range, it will start to spread," Offit said.

Tracking down Philadelphia's unaccounted kindergartners could prevent an outbreak.

But Offit said kids who don't get their shots in Philly public schools typically are not missing them on purpose, that parents aren't skipping vaccines by design.

"What I find, at least, people in the city believe in vaccines, they think they're a good thing," he said. "It's just sometimes difficult to get them," he said.

School nurses often are the best placed people to identify under-vaccinated students.

Today, Philadelphia's nurses can access the KIDS database, but KIDS and the state survey are not set up to share information.

Feemster sees room for improvement.

"That is an ideal, that a nurse could go to a city or a state's registry and say, 'I have data on this child, I don't have data on this one,'" she said.

At McKinley Elementary, Kelly said, too often, if kids don't get care from school nurses, they don't get it at all.

"We have kids that get hurt over the weekend and wait until Wednesday to show me. We have our frequent fliers, and we have kids that come to school very ill. They're not kept home when they should be."

Kelly said she's thankful to have access to the city database. But, if Philadelphia and the state could coordinate and share data, she said she could spend less time tracking down which kids have their shots and more time caring for them.

This is a corrected version, a previous version included an inaccurate statistic about McKinley's vaccination rate, though both numbers are significantly lower than the Philadelphia average.