Immune response to Haemophilus influenzae type b conjugate vaccine in preterm infants

Background: Haemophilus influenzae type b (Hib) vaccine became available for use in Japan in December 2008. The aim of the present study was to evaluate the immunogenicity of Hib vaccine in Japanese preterm infants. Methods: Serum samples were obtained from 54 preterm infants before the first vaccination and 1 month after the third. Anti‐polyribosylribitol phosphate (PRP) antibodies were measured using an enzyme‐linked immunosorbent assay method. Antibody positivity was defined as levels >1 µg/mL. Results: Of the 54 preterm infants, 46 (85.2%) achieved antibody levels >1 µg/mL. This compares with the 92.4% reported in full‐term infants. The antibody seroconversion rate of infants starting vaccination at 2 months of age was close to being significantly lower than when vaccination was started at 3 months of age (P= 0.060). In addition, the percentage of infants achieving a positive response in the group with a history of antenatal steroid exposure was significantly higher than in those not exposed (P= 0.046). Thus, risk factors for lower Hib antibody concentrations after three doses of vaccine were age at first vaccination and lack of use of antenatal steroids. Conclusions: There is a possibility that perinatal factors and the environment unique to preterm infants are related to their lower antibody positivity rates compared to full‐term infants. It may therefore be preferable to modify the proposed immunization schedule.