Siblings of patients with Haemophilus meningitis have impaired anticapsular antibody responses to Haemophilus vaccine.

Siblings of patients with type b Haemophilus influenzae meningitis are at increased risk of developing Haemophilus disease. We immunized 26 healthy siblings and 25 control subjects using a vaccine containing the type b polysaccharide capsule (10 micrograms PRP) and pertussis vaccine (4 opacity units) (Lederle Laboratories) to determine whether siblings of patients with Haemophilus meningitis had an impaired antibody response to PRP. Using two intramuscular injections one month apart, we found that the siblings had a lower response to PRP. One month after the second injection, 12 of 24 of the siblings had serum concentrations of anticapsular (PRP) antibody thought to be sufficient to confer protection against Haemophilus disease (greater than or equal to 300 ng/ml), compared with 19 of 24 of the control children tested (50% vs 79%, P = 0.035 by chi-square analysis). In comparison with the normal controls, the siblings produced significantly less IgG anti-PRP antibody but similar amounts of IgM. The impaired responsiveness to PRP was most evident among the 16 children born after their sibling had meningitis and who were not known to have been exposed to type b Haemophilus infection previously. These data indicate that siblings of some patients with type b Haemophilus meningitis have reduced ability to form IgG anti-PRP antibody, which may be associated with increased susceptibility to Haemophilus disease.