PROTECTION PROVIDED BY HAEMOPHILUS-INFLUENZAE TYPE-B CONJUGATE VACCINES IN LOS-ANGELES-COUNTY - A CASE-CONTROL STUDY

The objective was to assess the degree of disease control and to evalu ate the protective efficacy of licensed Haemophilus influenxae type b (Dib) conjugate vaccines (HbOC, PRP-OMP, PRP-D) used routinely in chil dren 2 to 35 months of age. We conducted a case-control study in Los A ngeles County between January 1, 1991, and December 31, 1992, and a co hort analysis of Dib cases between 1983 and 1992. For the case-control study 105 cases of invasive Hib disease were identified and 767 geogr aphically and age-matched controls were selected by random digit telep hone dialing. Sixteen HbOC vaccine failures occurred > 14 days after a single dose of vaccine, 6 vaccine failures after 2 doses and 3 failur es after 3 doses; 2 cases occurred 6 and 12 days, respectively, after an initial dose of HbOC. The protective efficacy of a single HbOC vacc ine dose was 71.1% (95% confidence interval (CI), 37.5 to 87.2%). Afte r 2 doses the efficacy was 88.8% (95% CI, 59.5 to 96.9%) and after 3 d oses it was 94.4% (95% CI, 68.0% to 99.0%). Similar 95% CIs were seen for 1 and 2 doses of PRP-OMP vaccine. Adjustment of efficacy estimates for potential confounding variables did not significantly alter the r esults. Despite relatively low rates of immunization (20 to 60%) the r ates of Hib disease decreased strikingly between 1990 and 1992 (from 2 4.2 to 4.4/100 000 children < 5 years of age). The HbOC conjugate vacc ine, used predominantly but incompletely during this period, provided substantial protection against invasive Dib disease in children immuni zed between 2 and 35 months of age. Optimal protection was afforded on ly after 3 doses of HbOC vaccine, but appreciable protection was affor ded by 1 or 2 doses. The decreased rates of Hib disease suggest that a reduced likelihood of disease was afforded to unvaccinated children a s well.