Long-term antibody levels and booster responses in South African children immunized with nonavalent pneumococcal conjugate vaccine.

Children who had initially received three doses of either a nonavalent pneumococcal conjugate vaccine containing serotypes 1, 4, 5, 6B, 9V, 14, 18C, 19F, and 23F or placebo at 6, 10, and 14 weeks of age were bled at 9 and 18 months for determination of antibody concentrations. The children were then randomized to receive a booster dose of either the 9-valent pneumococcal conjugate vaccine or a 23-valent polysaccharide vaccine and antibody levels determined 1 month later. At 9 months, the geometric mean concentrations (GMCs) were significantly higher for all vaccine serotypes in vaccinated children compared with controls (means varied from 0.49 microg/ml for serotype 4 to 2.37 microg/ml for serotype 14). At 18 months, antibody concentrations remained significantly higher in vaccinated children (means varied from 0.19 microg/ml for serotype 4 to 1.1 microg/ml for serotype 14). In children who had received conjugate vaccine in infancy, the conjugate vaccine at 18 months produced a significant booster response for serotypes 1, 6B, 14, 19F, and 23F (means varied from 2.74 microg/ml for serotype 19F to 15.52 microg/ml for serotype 6B) and produced a comparable response to a first dose of conjugate at this age for serotypes 4, 5, 9V, and 18C. Boosting at 18 months with polysaccharide vaccine produced higher antibody concentrations to all serotypes in children who had previously received conjugate vaccine compared to children who had not received the conjugate vaccine in infancy. In conclusion, the 9-valent pneumococcal conjugate vaccine given in infancy elicits significant and long-lasting antibody responses which can be boosted with either the conjugate or polysaccharide vaccines.