Serum antibody responses of weanling mice and two-year-old children to pneumococcal-type 6A-protein conjugate vaccines of differing saccharide chain lengths.

Type 6A pneumococcal polysaccharide (PS) fractions having mean lengths 2, 7, or 14 repeat units were coupled to tetanus toxoid, giving conjugates T-6A-2, T-6A-7 or T-6A-14. In weanling outbred mice, type-6A serum antibody responses were minimal to T-6A-7 but significantly higher to T-6A-2 and T-6A-14. In contrast, two-year-old humans responded increasingly to T-6A-2, T-6A-7, and T-6A-14; type 6B antibody rose, but variably. Upon challenge with 6B PS at age 3 yr, conjugate-primed children made anamnestic responses. T-6A-14, containing the longest saccharide chains (and highest saccharide:protein mass ratio) excelled in priming as well as antibody induction.