The agency expects the shortage -- which covers Merck's PedvaxHIB and Comvax (Hib/hepatitis B) shots -- to last well into next year. The recommendation was announced Wednesday in a CDC publication, Morbidity and Mortality Weekly Report: Dispatch.

"The booster dose is being deferred temporally in order to save vaccine," Allen explained. "The deferral of boosters will likely be many months, possibly until the end of 2008."

"Parents should continue to get the initial doses of the vaccine," Allen added.

Allen noted that this delay should not cause an increase in Hib-related infections, "primarily because of the herd immunity that is afforded by the number of children who are vaccinated." In 2006, 94 percent of all U.S. children had been vaccinated, Allen said, so the chances of Hib spreading to any one child remain low -- a phenomenon doctors call "herd immunity."

However, the agency is asking that physicians keep track of patients who did not get the booster shot so they can be notified when the shortage is over, Allen said.

"We are looking at a bubble in time where we have to accommodate what we are able to do," said Dr. Jonathan L. Temte, an associate professor of family medicine at the University of Wisconsin School of Medicine in Madison and a spokesman for the American Academy of Family Physicians.

"Parents who have had their children vaccinated within the past weeks or months should not be concerned," Temte said. "As far as we know, there is no evidence or reports of any untoward effect in a vaccine recipient."

Temte noted that because so many children have been vaccinated, "the likelihood of new disease taking off is very low."

The Hib vaccine is recommended for all children under 5 and is usually given in a three-shot series, starting at 2 months of age. The Hib booster is recommended for children at 12 to 15 months. The delay in getting the booster will continue until the vaccine supply improves, the CDC said.

There is one exception to the recommendation, however: The agency recommends that children at high risk for Hib disease continue to receive the booster dose. Children falling into that category include American Indians, Alaska Natives and children with conditions such as asplenia, sickle cell disease, HIV, immunodeficiency conditions and certain cancers.

The CDC took this action in consultation with the its Advisory Committee on Immunization Practices, the American Academy of Pediatrics and the American Academy of Family Physicians.

More information

For more about Haemophilus influenzae type B, visit the U.S. Centers for Disease Control and Prevention.