This article is from the Childhood Vaccinations FAQ, by Lynn Gazis-Sax firstname.lastname@example.org with numerous contributions by others.
Estimates from different labs vary a lot. AHFS Drug Information, after
noting this variability, and the uncertainty as to what antibody level
is adequate for protection, says that in one study, 75% of children
18-23 months and 85% of children 24-29 months had serum anticapsular
antibody levels of one microgram per milliliter or greater. The PDR
lists numerous studies, with results ranging from 100% efficacy to one
study in which vaccinated children had more cases of HiB than the
unvaccinated group. With the exception of the latter study, all of the
studies showed significant positive results, often with efficacy
estimates over 90%.
According to a NY Times article of 12/18/90, HiB vaccine produces
lower antibody response among Native Americans, but the new conjugated
vaccine seems to produce higher antibody response in Native American
children and may protect all children at a younger age.