The dramatic decline in the incidence of invasive Hib disease, despite less than complete immunization, is likely due to a reduction in nasopharyngeal carriage. Studies in the U.S. and in Finland have demonstrated decreased Hib colonization after vaccination. However, the mechanism for this effect, as well as its relevance for other bacterial vaccines, remains unclear. To study the potential role of mucosal immunity, we evaluated antibody production in serum and nasal wash specimens among infants randomized to received PRP-OMP at 2, 4 and 12 months of age or PRP-T at 2, 4, 6 and 12 months of age. All specimens were analyzed by ELISA to detect serum IgG or mucosal IgA (mIgA). To control for variable nasal wash concentrations, mIgA activity are presented as a ratio of anti-PRP mIgA to total mIgA levels. Mucosal IgG levels are pending. Serum and mucosal responses to Hib conjugate vaccination are shown below:Table With either PRP-OMP or PRP-T, both serum and mucosal anti-PRP titers increased after immunization. We found no significant difference in serum anti-PRP antibody response to immunization with PRP-OMP or PRP-T, but mucosal levels were higher with PRP-T.

Table 1
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These data demonstrate an increase in anti-PRP IgA activity in the nasopharynx of infants immunized with Hib conjugate vaccines. In a previous study, we found that the predominant anti-PRP antibody in nasal wash specimens was IgG. This may explain the observed decrease in Hib colonization. These studies suggest that transudated IgG and locally produced IgA following immunization confer protection against respiratory bacterial pathogens.