they do not develop an immune response to subsequent vaccines given later in life.

According to Losonsky, 98 percent of full-term infants achieve protective immune responses after receiving the third dose of HBV (Losonsky, unpublished data). A study of HBV given to premature infants, however, found that only 54 percent of infants with birth weights of less than 1,000 grams and 70 percent of infants with birth weights of 1,000 to 1,500 grams attained protective immune responses after receiving the third dose of the vaccine. Thirteen of the premature infants with little or no detectable antibody responses to the three doses of HBV were given a fourth dose of the vaccine at 9 to 12 months of age. Only 46 percent of these revaccinated infants achieved protective antibody levels (Losonsky, unpublished data).

These data suggest, Losonsky noted, the possibility that tolerance develops in some premature infants. Researchers are following those infants who did not respond to the fourth HBV dose to see if their lack of response is permanent. Other studies suggest that the immune systems of premature infants given DPT and oral polo vaccine at standard times respond as well as those of full-term infants (Koblin et al., 1988; Conway et al., 1993; D'Angio et al., 1995). However, a decreased immune response to some enhanced-potency inactivated polio-oral polio vaccine combinations and to certain Haemophilus influenzae type b conjugate vaccines was seen in premature infants. The response depended on gestational age and weight at the time of the first immunization (Munoz et al., 1995; D'Angio et al., 1995).

According to Losonsky, these findings suggest that vaccine schedules, dosages, and combinations for preterm infants may have to differ from those for full-term infants. She added that the best schedule and dosage for each vaccine cannot be predicted on theoretical grounds but can only be determined by further study. Care must be taken, she said, not to leave premature infants vaccinated but unprotected from disease. A representative of the National Institute of Allergy and Infectious Diseases (NIAID) pointed out, however, that the risk of inducing tolerance to vaccination for a specific disease must be balanced with the risk of the infant dying or suffering long-term consequences from natural infection. Premature infants, for example, are more likely to have respiratory abnormalities that may put them at greater risk for complications from the respiratory disease pertussis (whooping cough).

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