Cross-Immunogenicity of Pneumococcal Group 9 Capsular Polysaccharides in Adult Volunteers

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Abstract
Group 9 organisms (types 9N, 9A, 9L and 9V) account for about 3-4% of pneumococcal disease isolates throughout the world. Types 9N and 9V comprise about 90% of the group 9 disease isolates. Type 9N is more common than type 9V in adults; type 9V predominates in infants and children. In the USA there have been 8 reported cases due to group 9 pneumococci in individuals previously vaccinated; 6 were type 9V and 2 were type 9N. To ascertain the cross-immunogenicity of group 9 polysaccharides, volunteers were injected with vaccines of monovalent types 9N, 9A, 9V or 9L, or bivalent (9N and 9A) or trivalent (9N, 9A and 9V) polysaccharide vaccines. Monovalent types 9N, 9V and 9L each stimulated a 5.8- to 7.5-fold geometric mean rise; at least 80% of the volunteers responded with a 2-fold or greater homologous antibody rise. Type 9V induced a 5.8-fold geometric mean rise, but only 66% of the volunteers responded with a 2-fold or greater homologous antibody rise. Type 9N induced only a 2.1-fold geometric increase; only 54% of the volunteers responded with a 2-fold or greater rise in anti 9V antibodies. Types 9L and 9A were the most cross-immunogenic. The trivalent preparation (9N, 9A and 9V) gave the highest geometric mean titer and seroconversion rate to each of the group 9 polysaccharides. Thus, the polyvalent pneumococcal vaccine with its type 9N evidently does not induce a satisfactory anti-type 9V response and should contain additional components in order to achieve greater protection against group 9 organisms.

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