Cross-reacting opsonic antibodies to clinically important pneumococcal serotypes after pneumococcal vaccination.

Opsonic activity of serum to pneumococcal serotypes 6 B, 9 V and 19 A was measured in 16 patients before and after immunization with a pneumococcal vaccine. The capsular polysaccharides of these serotypes are not included among, but are antigenically related to the vaccine polysaccharides. Patients responding to immunization with a twofold increase in serum antibodies to vaccine polysaccharides 6 A, 19 F and 23 F were studied. Increased opsonic activity towards serotypes 6 B, 9 V and 19 A was found in 12, four and ten patients respectively. In ten of the patients antibodies to serotypes 6 B, 9 V and 19 A were measured by a staphylococcal protein-A binding assay. A twofold increase in antibodies was found in postvaccination samples from ten, three and seven patients respectively. These results indicate that humans responding to pneumococcal vaccination, may also develop opsonic antibodies to other clinically important pneumococcal serotypes. The degree of cross-immunization appears to vary between individuals and between different pneumococcal serotypes.