Meningococcal Group C and W135 Immunological Hyporesponsiveness in African Toddlers

A phase II clinical study was conducted in African toddlers (aged 12 to 23 months), with subjects receiving either investigational meningococcal group A conjugate (PsA-TT), meningococcal ACWY polysaccharide (PsACWY), or Haemophilus influenzae type b (Hib-TT) vaccine. Ten months following vaccination, the 3 study groups were further randomized to receive a dose of PsA-TT, a 1/5 dose of PsACWY, or a dose of Hib-TT vaccine. Group A serum bactericidal antibody (SBA) results have been reported previously, with PsA-TT demonstrating superior immunogenicity versus PsACWY vaccine. Immunogenicity for serogroups W135 and C was assessed by SBA assay to investigate the impact of multiple doses in this age group. Blood samples were taken prior to vaccination, 28 days and 40 weeks post-primary vaccination, and 7 and 28 days post-booster vaccination with a 1/5 dose of PsACWY. Subjects who had previously received a full dose of PsACWY had W135 SBA geometric mean titers (GMTs) of 26.1 and 4.4 at 7 and 28 days post-booster vaccination with a 1/5 PsACWY dose, respectively, whereas the W135 SBA GMTs of naive subjects at these time points following vaccination with a 1/5 dose of PsACWY were 861.1 and 14.6, respectively. Similar differences were observed for serogroup C, with SBA GMTs of 99 and 5.9 at 7 and 28 days post-booster vaccination with a 1/5 dose of PsACWY, respectively, for naive subjects, compared to 4.1 and 3.2 for previously vaccinated subjects. Immunologic hyporesponsiveness for groups C and W135 was observed following a full dose of PsACWY vaccine at 12 to 23 months of age and a 1/5 dose of PsACWY 10 months later compared to the case for PsACWY-naive subjects receiving a 1/5 dose of PsACWY vaccine. Outbreaks of Neisseria meningitidis group A recur frequently in the African meningitis belt, where they are responsible for high mortality and morbidity. Polysaccharide vaccines against group A have been available since the 1970s, but due to the limitations of these vaccines, polysaccharide-protein conjugate vaccines have been developed. In 2001, the Meningitis Vaccine Project (MVP), a partnership between the World Health Organization (WHO) and the Program for Appropriate Technology in Health (PATH), was created through core funding from the Bill & Melinda Gates Foundation, with the goal of eliminating group A meningococcal epidemics in sub-Saharan Africa through the development and use of a monovalent group A meningococcal conjugate vaccine. A phase I clinical study of a group A meningococcal conjugate vaccine, MenAfriVac