An outbreak of Shigella sonnei infection was discovered in two adjacent wards of a custodial institution shortly after oral streptomycin-dependent S. sonnei and Shigella flexneri 2a vaccines had been administered to newly admitted children as part of a field trial of vaccine efficacy. An investigation was conducted because of concern that the vaccine strain might have reverted to a virulent state and caused clinical disease. The wild strain isolated from children with disease differed from the vaccine strain by its invasiveness in the guinea pig eye model, antibiogram, and colicin type. The outbreak was traced to a recently admitted child who apparently introduced the wild strain 11 days before administration of vaccine.
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Levine et al. (1974) studied this question.