Anticapsular polysaccharide IgG concentrations in Mexican children formerly immunized with Haemophilus influenzae b PRP-T vaccine.

BACKGROUND: In 1999 H. influenzae b (Hib) PRP-T vaccine was introduced into primary immunization schedule in Mexico. There have been no studies evaluating antibody response after widespread immunization in our country. It is now recognized that Hib conjugates induce significant initial antibody levels that in some cases wane over time. This study relies on the measurement of IgG serum antibody concentrations to Hib capsular polysaccharide (PS) and is interpreted in the light of the accepted levels > or =0.15 microg/mL for short-term protection against Hib invasive disease and > or =5.0 microg/mL for protection against Hib oropharyngeal carriage.
METHODS: Using a validated ELISA assay, we measured the IgG serum antibody concentrations in 115 children between 7 and 93 months of age who had received three doses of PRP-T. We used the standard reference serum US FDA 1983 for quantification of PS antibody levels. Concentrations were estimated using 3(rd) degree polynomial regression lines. As there was no unvaccinated group available (>95% of Mexican children have received the Hib vaccine), the study was designed as a cross-sectional.
RESULTS: All children had serum IgG concentrations > or =0.15 microg/mL [range 0.24-54.64 microg/mL]; 69.6 % (80/115) had > or =1.0 microg/mL and 14 % (16/115) showed concentrations > or =5.0 microg/mL. The vaccine elicited geometric mean concentrations (GMC) of 4.0, 1.6, 1.4 and 2.4 microg/mL in groups of 7-12, 13-24, 25-48 and 49-93 month-old respectively.
CONCLUSIONS: PRP-T vaccination in this group of Mexican children has resulted in serum IgG concentrations > or =0.15 microg/mL, suggesting that Hib immunization has conferred protection against invasive disease.