ABSTRACT
There is also evidence for heterologous serotype protection in humans. Sera from subjects who had previously received a S. flexneri 2a vaccine or wild-type challenge strain cross-reacted with serotype 2b (which shares a type II antigen), and serotypes 1a, 5a, and Y (and, to a lesser extent, 3b and 4a), which share group 3,4 antigens with 2a (14). Immunoglobulin A (IgA) was the predominant cross-reacting antibody. A field trial evaluating the efficacy of streptomycindependent (SmD) S. flexneri 2a vaccine concluded that protection was limited to the homologous serotype, suggesting that experimental cross-reactivity did not translate into clinical immunity (9). However, the attack rate of S. flexneri serotypes not in the vaccines was too low to assess heterologous immunity definitively.
