RÉSUMÉ
In this study we compared natural vs. induced Haemophilus influenzae type b (Hib) anti-capsular polyribosylribitol phosphate (PRP) antibody response in a low socioeconomic population. One hundred twenty five 2-month-old children received the complete HbOC vaccine immunization scheme and a booster dose at 15 months of age. One hundred twenty five non-immunized children served as the control group. Serum Hib anti-PRP antibody titers were determined by ELISA in all children. We found at the end of the primary immunization scheme an antibody concentration of 27.28 micrograms/ml in the immunized group vs. 7.48 micrograms/ml in the control group. The antibody response was mainly of the IgG1 class in both groups. After the booster dose the antibody concentration was 30.14 g/ml in the vaccinated group vs. 6.06 micrograms/ml in the control group (p < 0.01). Ninety nine percent of immunized and non-immunized infants had titers greater than 1 microgram/ml. These results confirm that immunization with the HbOC vaccine induces an important increase in anti-PRP specific antibody titer, but they also demonstrate that natural exposure induces responses higher than those referred as protective (1 microgram/ml).