Study leader Elizabeth Luman of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention said that more than one in four children are out of compliance with U.S. vaccination guidelines

For 50 years, the U.S. government has counted missed vaccine doses to measure compliance, but that standard significantly underestimates the problem. Government figures do not account for other vaccine lapses -- like doses given at the wrong age or at wrong intervals. By factoring in these lapses, the study found that compliance drops to 72 percent -- 9 percent lower than previously thought.

The study, scheduled to be published in the June issue of the American Journal of Preventive Medicine, found missed doses account for two-thirds of vaccine lapses.

The study also revealed that some children get their shots too early. For instance, about 3 percent of children had their last hepatitis B vaccines prior to age 6 months, the minimum recommended for that immunization. Other toddlers received their first measles vaccine while their mother's antibodies still protected them -- in effect wasting that dose.