Infants were immunized with 1 of the 3 experimental pneumococcal conjugate
vaccines that contain 6B and 19F but not 6A or 19A serotypes, Their sera we
re studied for the capacity to opsonize Streptococcus pneumoniae 6A, 6B, 19
A, and 19F serotypes and the level of IgG antibody to the 4 serotypes, Sign
ificant increases were observed in the number of infants with detectable op
sonophagocytic titers with 3 conjugate vaccines for 6B (vaccine) serotype b
ut with only 2 vaccines for 6A (cross-reactive) serotype, Significant incre
ases were observed with 2 conjugate vaccines for 19F serotype but with only
1 vaccine for 19A serotype, Thus, some conjugate vaccines may elicit cross
-protection better than others. In addition, correlations between opsonopha
gocytic titers and IgG antibody levels by ELISA were high for 6B and 19F se
rotypes but low for 6A and 19A serotypes, Thus, ELISA may be an inadequate
surrogate assay of vaccine response for cross-reactive serotypes.