1 in 4 U.S. Toddlers Improperly Vaccinated

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For this study, researchers looked at vaccination histories for more than 17,500 U.S. children aged 19 months to 35 months.

An estimated 72 percent of children in this age group finished the standard vaccination series. That was 9 percentage points lower than if coverage was based on counting doses only, the researchers noted.

Nineteen percent of children were missing one or more doses of vaccines; 8 percent 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 (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 too close together.

"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 lack of strict adherence to the vaccine schedule may be a fading consciousness among today's parents of what these immunizations are protecting children against. Frenck said he remembers seeing a childhood friend in an iron lung, the result 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."

So far, smallpox has been completely eradicated from the planet thanks to immunizations, while great gains have been made with measles and polio.

"People just need to keep their vigilance up," Frenck 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."

More information

For more on vaccines and vaccine schedules, visit the CDC's Advisory Committee on Immunization Practices.