Responses to immunisation with Hib conjugate vaccine in Australian breastfed and formula-fed infants

Objective: There are conflicting reports as to whether breastfed infants respond with higher antibody levels to conjugate Haemophilus influenzae type b (Hib) vaccine compared with formula-fed infants. These observations prompted us to investigate the effect of feeding method on the antibody concentration to Hib polyribosylribitol (PRP) both prior to and 3 months after the primary course of immunisation with Hib (PRP-OMP). Methods: We measured plasma concentrations of IgG antibody to Hib PRP by enzyme-linked immunosorbent assay in blood samples from a total of 272 breastfed and formula-fed infants prior to immunisation (7 weeks of age, n = 82 and n = 148, respectively) and again 3 months after completion of the primary course of immunisation with Hib PRP-OMP (7 months of age, n = 88 and n = 132, respectively). Results: Breastfeeding was associated with lower plasma antibody titres at both times (P < 0.01, T-test) with 49% of breastfed infants having anti-PRP concentrations below 1.0 mu g/mL at age 7 months. There was no reported invasive Hib disease in this cohort of infants, and nationally the effectiveness of the Hib vaccination programme remains high. Conclusions: These data suggest that breastfeeding may be associated with immunomodulation of infant Hib immunisation responses with this immunisation regime. Further research is needed to determine whether differences in antibody concentration described here are primarily determined by factors directly attributed to breastfeeding or whether other environmental factors may play a significant role.