New numbers show that more than a quarter of American toddlers may not have received their proper vaccinations.

A new study of children aged 19 months to 35 months found that missed doses account for about two−thirds of non−compliance to official recommendations. However, mistimed doses are also a contributing factor.

...[b]ased on data from 2003 and 2004, a time when a toddler up to 18 months old should have received about 14 shots of several different vaccines. Today, even more shots are recommended.

Partially or completely skipping immunization puts children at risk for a variety of terrible — and preventable — diseases such as measles, mumps and chicken pox.

"This is a little bit of a wake−up call — not a huge one — that you need to make sure to do the best you can to get children vaccines when they're supposed to get them," added Dr. Paul Offit, director of the Vaccine Education Center and chief of infectious diseases at the Children's Hospital of Philadelphia. He stressed that "kids may catch up [when they're older]."

To be fair, today's families face a complicated schedule of vaccinations in the first years of life. This study was based on data from 2003 and 2004, a time when a toddler up to 18 months old should have received about 14 shots of several different vaccines. Today, even more shots are recommended.

"[But] the official recommendations for vaccination include more than just number of doses," said Elizabeth Luman, lead author of the study and an epidemiologist at the CDC's National Center for Immunization and Respiratory Diseases in Atlanta. They also include specific age recommendations and multiple doses at different time intervals.

For this study, researchers looked at the vaccination histories of more than 17,500 U.S. children aged between 19 and 35 months.

An estimated 72 percent of children in this age group finished the standard vaccination series. Nineteen percent of the children were missing one or more doses of vaccines and 8 % had received an "invalid" dose, meaning it was given while the child was too young or too close to the previous dose.

About 3 percent of the sample had their last hepatitis B vaccine too early in life, (i.e., before six months). Some also received their measles vaccine while they were still protected by their mother's antibodies. Another 3 percent received serial doses of one vaccine without waiting the recommended interval of time.

"If children receive vaccines too close together or too early, they're not as likely to be protected, and if you have a lot of that, then you're more likely to have disease outbreaks," Luman said.

One reason for this lack of strict adherence to vaccine schedules may be that today's parents cannot remember what the immunizations are protecting children against. The CDC's Robert Franck, M.D., described seeing a childhood friend in an iron lung because of polio.

"It scared me to death," he said. "Kids these days, and probably most adults, don't even known what an iron lung is — and that's because of immunization."

"People just need to keep their vigilance up," Franck said. "We need to continue to review shot records and to go over it with parents whenever they come in. Opportunities for vaccination are missed a lot of times when kids come in for one reason or another, and we don't look at the immunization record. We need to continue to try to immunize kids whenever we have the opportunity."

This study was published in the June issue of the American Journal of Preventive Medicine.