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Depression of anticapsular antibody after immunization with Haemophilus influenzae type b polysaccharide‐diphtheria conjugate vaccine

Invasive Haemophilus influenzae type b infections have been observed in the week after immunization with capsular polysaccharide vaccine. We sought to document depression of antibody concentrations after immunization of 18-month-old infants with H. influenzae type b capsular polysaccharide-diphtheria conjugate vaccine. All 9 infants with detectable preimmunization anticapsular antibody had depression of antibody concentrations on the second day after immunization (P = 0.002). By Day 7 all had achieved anticapsular antibody concentrations >0.15 μg/ml, a level believed to provide protection to immediate challenge with H. influenzae type b. Of those without detectable preimmunization antibody, 7 of 21 (33%; 95% confidence interval, 11 to 56%) had not achieved concentrations of >0.15 mg/ml 1 week after immunization. We conclude that there is depression of anticapsular antibody concentrations during the first week after immunization with H. influenzae type b capsular polysaccharide-diphtheria conjugate vaccine. We speculate that H. influenzae type b infections after immunization with H. influenzae type b vaccines may be the result of: (1) low antibody concentrations because of either depression of antibody concentrations or failure to develop antibody; and (2) exposure to H. influenzae type b. Depression of antibody concentrations could be explained by binding of in vivo antibody to the vaccine.