Naturally acquired and conjugate vaccine-induced antibody to Haemophilus influenzae type b (Hib) polysaccharide in Malian children: serological assessment of the Hib immunization program in Mali

Abstract

Haemophilus influenzae type b (Hib) conjugate vaccine for infants (6, 10, and 14 weeks of age) was introduced into the Malian Expanded Program on Immunization in July 2005, to diminish invasive Hib disease in young children. Antibodies to Hib capsular polysaccharide (PRP) were measured in infants and toddlers from an area already served by the Hib immunization program (Bamako) and in unimmunized children of the same age in a district (Kangaba) where Hib immunization had not yet begun. Among vaccinated Bamako children 6-23 months of age, 77-93% exhibited PRP titers ≥ 1.0 μg/mL, indicating long-term protection, versus only 10-23% of Kangaba children of that age. High PRP antibody titers in immunized children persisted through 2 years of age. Moreover, ∼50% of Bamako children exhibited anti-PRP titers ≥ 5.0 μg/mL; a level that impedes Hib upper respiratory carriage, and may thereby diminish the Hib transmission to the unimmunized susceptible population (i.e., providing indirect protection).

Figure 1.

Geometric mean titer (GMT) and 95% confidence interval of serum IgG anticapsular polysaccharide antibody in children from rural Kangaba (Hib unvaccinated) and urban Bamako (Hib vaccinated) by age group. The GMTs for the 6-week age groups represent pre-vaccination immune status and were not significantly different. The GMTs for all other age groups were significantly higher in Bamako than in Kangaba (one-sided P < 0.025, Wilcoxon rank sum test). The number of children per age group per site was 30.