Field Trial of Attentuated Salmonella Typhi Live Oral Vaccine Ty21a in Liquid and Enteric-Coated Formulations and Epidemiological Survey for Incidence of Diarrhea due to Shigella Species

In October, 1986, a randomized, controlled field trial was initiated in Area Sur Oriente and Area Norte, Santiago, Chile, to compare the relative and absolute efficacy of three doses of Ty21a vaccine given in enteric-coated capsule or liquid formulation. Intensive clinical and bacteriologic surveillance is being maintained. Preliminary data show that the liquid formulation is associated with a significantly lower attack rate for typhoid fever than the enteric-coated capsule formulation. Surveillance for this trial continues. Epidemiologic and microbiologic surveillance was initiated in three populations of children in the Santa Julia neighborhood of Area Oriente, Santiago with the broad objective of preparing a field area where the efficacy of vaccines against Shigella, enterotoxigenic E. coli and other diarrheal pathogens can be evaluated. In the first year of surveillance diarrhea in young children showed a prominent seasonality with approximately twice as many episodes occurring in the warm season. Similarly, isolation of Shigella was maximal during the warm months. An impressive portion of the episodes of diarrhea in Santa Julia children particularly during summer, are due to Shigella and E. coli pathogens. The high incidence rates of Shigella, ETEC, and EPEC infection make Santa Julia a suitable place for testing the efficacy of vaccines against these agents.