1 in 4 U.S. Toddlers Improperly Vaccinated

"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.

SOURCES: Elizabeth Luman, Ph.D., epidemiologist, National Center for Immunization and Respiratory Diseases, U.S. Centers for Disease Control and Prevention, Atlanta; Robert Frenck, M.D., professor, pediatrics, Cincinnati Children's Hospital, and member, American Academy of Pediatrics' committee on infectious diseases; Paul A. Offit, M.D., director, Vaccine Education Center, and chief, infectious diseases, the Children's Hospital of Philadelphia; June 2008American Journal of Preventive Medicine