Shigellosis in custodial institutions. V. Effect of intervention with streptomycin-dependent Shigella sonnei vaccine in an institution with endemic disease.

A double-blind controlled field trial of live, oral, streptomycin-dependent Shigella sonnei vaccine was begun in an institution with endemic S. sonnei disease. Considerable unexpected child-to-child transmission of the vaccine strains inadvertantly caused the field trial to resemble a mass vaccination campaign. Although S. sonnei accounted for 90% of shigella infections from 1968 to 1971 and three-fourths of the cases occurred in the seven study cottages, S. sonnei disease disappeared following vaccination; epidemiologic features suggest a causal relationship. Clinical S. sonnei disease did not occur despite the detection by bacteriologic surveillance of carriers of virulent S. sonnei. Levels of hygiene remained compatible with transmission of shigella since 43 cases of S. flexneri 6 were seen. If the interpretation is correct, the disappearance of S. sonnei disease resulted from inadvertent "mass vaccination" and oral shigella vaccines may prove useful for control of endemic shigellosis institutions. Nevertheless, a properly designed controlled field trial, taking into account transmissibility of vaccine, in an institutional setting similar to Sunland is necessary to substantiate the role of oral shigella vaccines in control of institutional shigellosis.