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Cell proliferation index measured before and after booster immunization. The PA-based vaccine was injected (in 0.5 ml s.c.) into guinea pigs, and a booster of the same PA dose was given 8 weeks after the primary immunization. Cell proliferation assays and anti-PA titer ELISAs were performed 14 days after primary and booster immunization. (A) 1-μg dose prebooster; (B) 1-μg dose postbooster; (C) 25-μg dose prebooster (positive control); (D) negative control.
