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April 29, 2008 -- Many young children in the United States have been under- or improperly vaccinated, and vaccine coverage rates are lower than previously reported as a result, the CDC says.
Using a modified method for calculating vaccine coverage, CDC researchers concluded that more than one in four babies and toddlers are out of compliance with government vaccination guidelines.
Instead of an 81% coverage rate among children aged 19-36 months, as has previously been reported, only about 72% of children in this age group have been adequately immunized, the CDC researchers conclude.
One problem is that many babies and young children who are vaccinated get their shots too early or too close together, CDC epidemiologist Elizabeth Luman, PhD, tells WebMD.
"If you get a vaccine too early or get doses too close together, they aren't as effective. They are not considered valid doses, and should be repeated," she says. "Our study found that this is happening quite often."
The researchers analyzed the vaccination histories of about 17,500 children between the ages of 19 and 35 months whose parents took part in a 2005 nationwide immunization survey.
The aim was to determine if government vaccination recommendations were being followed.
Guidelines established by the Advisory Committee on Immunization Practices (ACIP) recommend which vaccines should be given and in what doses, at what age children should get the vaccines, the minimum age at which doses are considered valid, and the minimum time between doses within a vaccination series.
Previous vaccine coverage calculations have been based solely on the number of doses given, Luman tells WebMD.
When ACIP guidelines were considered, 19% of the children included in the survey were missing one or more doses of the recommended series of vaccinations and 6% had received at least one early, invalid vaccination.
Fourteen percent of the children missed the fourth dose of the diphtheria, tetanus, and pertussis vacine (DTaP); 9% had not been vaccinated against measles, mumps, and rubella (MMR), and 8% had not had a third dose of the polio vaccine.
About 14% of children had their third dose of the hepatitis B vaccination too early, and some toddlers received their first dose of measles vaccine while still protected by their mother's antibodies.
The report called on parents and pediatricians to follow ACIP guidelines, but it also recognized that vaccine providers must also balance the need to administer valid vaccinations with the reality of missing opportunities to vaccinate.
"We know there is a balance between missing opportunities to vaccinate and getting the timing just right," says CDC director of the immunization services division Lance E. Rodewald, MD. "But you don't want to vaccinate too late because that leaves the child unprotected longer, and you don't want to vaccinate too early because there may not be an optimal response."
Rodewald says the study should serve as a reminder of the importance of timing for administering vaccines.
The study appears in the June issue of the American Journal of Preventive Medicine.
The goal is to vaccinate every baby born in the United States against 14 preventable diseases before age 2.
Despite the study findings, Rodewald says vaccination rates are still high among young children in the United States.
"Coverage has never been higher, and we want to keep it that way," he says.