Nearly one in five North Carolina toddlers fails to get properly vaccinated against childhood diseases, says a new study from the Centers for Disease Control and Prevention.

State health officials point out that North Carolina's compliance rate is higher than all other states except Massachusetts. But with the rapid rebound of childhood diseases once thought to be in firm check, children whose vaccines and booster shots aren't properly administered can face a greater risk of preventable illnesses such as measles, mumps, whooping cough and polio, the CDC study says.

"We stand by our record," said Dr. Leah Devlin, state health director, while acknowledging the study's value for further improvement. "We really do a good job on immunization, between the health departments and the pediatric providers."

The new study, which measured compliance according to strict deadlines for vaccines and booster shots, found that among North Carolina children age 19 months to 35 months, only 81 percent were fully immunized against a range of childhood diseases. The study also indicates that the daunting schedule of shots necessary to make their children immune to this array of illnesses may cause parents to skip them.

When children aren't immunized properly, it increases the risk of infection. The CDC reported Thursday that the number of measles cases last year in the United States was the highest in five years. Some infections occurred in babies who had been exposed to the virus before they were eligible for vaccination, but most involved those who had not been properly vaccinated.

While some parents opt out of vaccinations for religious or other reasons, a growing force behind the failure to comply with vaccine schedules is the complexity of the regimen.

"Over the past few years, the vaccination schedule has become increasingly complex with the addition of new vaccines and vaccine-combination options," according to the report, published this week in the American Journal of Preventive Medicine.

A chart for immunizations on the CDC Web site lists dozens of shots for 13 different diseases scheduled from birth to age 6, with different color codings, footnotes and exceptions. A separate chart follows for children up to age 18.

Jennifer Gilmore, a Cary mother of two, said she relies on her pediatrician to know when her children are due for a shot.

"Sometimes they'll say, 'We need to get this one done now and that one can wait,' " Gilmore said, noting that it still takes diligence to get that advice in a timely fashion. "You have to show up for every one of those well-child visits to make sure they get what they need."

In North Carolina, vaccinations against 10 diseases are required for admission to day-care centers and schools; the state pays for the shots, although doctors often charge a visit fee. But parents face the ultimate responsibility of making sure children get their shots at the right time. Frequent moves and changing caregivers can add challenges to meeting the vaccination schedules.

"When there are other caregivers, like grandparents, or a change in custody, that's kind of a red flag that a child may be behind in vaccinations," said Dr. Lauren Lingley, who sees children and other patients at the new UNC family-medicine center in Hillsborough. "We make every effort to catch children up at the recommended intervals when we find that we have missed the correct times, or more often it's missed doses."

But playing catch-up can mean the immunization may not be as protective, the new study contends.

"Mis-timed vaccinations can lead to the reduced immunity of individuals and increased levels of population susceptibility," the report said.

The CDC's goal is to make sure that children get the most possible benefit from the vaccinations and that other people in the community don't catch one of the diseases.

Local pediatricians said that some parents mistakenly elect not to have their children vaccinated because of "social pressure," including accounts of potential harm from vaccines. Many parents fear the vaccine for measles, mumps and rubella is linked to autism -- a contention doctors say has not been scientifically proven.

"Missed shots are the things that we really need to tackle," said Dr. Dennis Clements, chief of primary care pediatrics at Duke Children's Hospital.

In addition to the measles outbreaks, which the CDC says are ongoing in Wisconsin, Arizona, Michigan and New York, there has been an uptick in pertussis, also known as whooping cough. Cases nearly doubled in North Carolina in one year, rising from 127 confirmed cases in 2005 to 237 in 2006. Doctors now recommend that adults and adolescents get a tetanus-diphtheria-pertussis booster, called TDAP, instead of the tetanus-diphtheria booster.

"I think it's really important for parents to be reminded that they need to complete the series of vaccinations," Devlin said. "Get that last shot of the series."