Duration of efficacy of Ty21a, attenuated Salmonella typhi live oral vaccine

Currently, two different formulations of Ty21a live oral typhoid vaccine ar e commercialized. The enteric-coated capsule formulation was licensed based on results of three years of follow-up of a randomized, placebo-controlled , double-blind field trial in Area Occidente, Santiago, Chile, which demons trated that three doses of this formulation, given on an every other day im munization schedule, conferred the best protection among several options ev aluated. Subsequently, a liquid formulation (lyophilized vaccine organisms reconstituted with buffer and water into a vaccine cocktail) was commercial ized after it was shown to provide superior protection than enteric-coated capsules over three years of follow-up in a randomized, placebo-controlled field trial in Area Sur Oriente and Area Norte, Santiago. Surveillance in t he Area Occidente trial was continued for four additional years (i.e., tota l seven years of follow-up) and in the Area Sur Oriente/Area Norte trial fo r two additional years (i.e., a total of five years of follow-up). These ad ditional surveillance data, which were analyzed to ascertain the longevity of protection conferred by these formulations of Ty21a, revealed that three doses of Ty21a in enteric-coated capsules (every other day schedule) confe rred 67% protection over three years and 62% protection over seven years of follow-up, whereas three doses of liquid formulation (every other day sche dule) elicited 77% protection over three years and 78% over five years of f ollow-up. Based on its excellent clinical acceptability, ease of oral admin istration, proven practicality in school-based mass immunization, and long- term efficacy enduring at least seven years, it is proposed that school-bas ed immunization with Ty21a be utilized as a control measure in areas where the incidence of typhoid fever is high and Salmonella typhi are antibiotic- resistant. (C) 1999 Elsevier Science Ltd. All rights reserved.