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Three injections of tetravalent pneumococcal polysaccharide-tetanus toxoid conjugate vaccine (PncT) were given to infants at 2, 4 and 6 months of age simultaneously with diphtheria–tetanus–pertussis and Haemophilus influenzae type b–tetanus toxoid conjugate vaccines. Three doses (1, 3 or 10 μg) of polysaccharides were used. Children were boosted with unconjugated polysaccharide vaccine at 14 months of age. No dose dependency was seen after primary immunization. However, booster response to three vaccine serotypes was highest in the group primed with the lowest dose of conjugate vaccine. As the magnitude of the response to booster may be related to the number of polysaccharide-specific memory B cells, we hypothesize that the 10 μg dose of the tetravalent conjugate vaccine is too high for optimal induction of immunologic memory.

We evaluated the effect of aging on the functional activity of naturally acquired anti-pneumococcal antibodies, the function of neutrophils in phagocytic killing of opsonized pneumococci, and the complement activity. Opsonic activities of antibodies to all tested pneumococcal serotypes were significantly lower and phagocytic killing of pneumococci by neutrophils was significantly impaired among the elderly, whereas the complement activity was slightly higher in the elderly than in the young adults. The reduced functional activity of serotype-specific antibodies and the compromised function of neutrophils in the opsonophagocytosis of pneumococci are likely to contribute to the increased susceptibility of the elderly to pneumococcal diseases.