In a 2003 survey, 37 percent of children in the US were undervaccinated for more than 6 months during their first 2 years of life, according to a new report.

Moreover, about 25 percent of children were delayed in receiving at least four vaccines, Dr. Elizabeth T. Luman, from the Centers for Disease Control and Prevention in Atlanta, and colleagues note in this week’s Journal of the American Medical Association.

“These results confirm that opportunity exists for improvement in vaccine administration in the US to ensure that all children remain fully vaccinated and optimally protected from vaccine-preventable diseases throughout early childhood - the time when children are most at risk for illness and severe complications from many vaccine-preventable diseases,” the researchers write.

The new findings come from the 2003 National Immunization Survey, which involved random contact with households and providers to estimate vaccination coverage rates for children. The analysis focused on 14,810 children between 2 and 3 years of age.

In addition to the 37 percent of children undervaccinated for more than 6 months, about 34 percent were unvaccinated for less than 1 month and 29 percent for 1 to 6 months, the investigators found.

Rates of undervaccination for more than 6 months varied greatly by vaccine type, ranging from 9 percent for poliovirus vaccine to 21 percent for Haemophilus influenzae type b (Hib) vaccine.

Roughly 21 percent of children were severely delayed in receiving their vaccines, meaning that they were undervaccinated for more than 6 months and missed at least four vaccines.

Factors linked to not receiving vaccinations on time included having an unmarried mother, living in a household with at least two children, black ethnicity, having two or more vaccination providers, and using public vaccination providers.

SOURCE: Journal of the American Medical Association, March 9, 2005.