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Shigella conjugate vaccines have been shown to be safe, immunogenic and efficacious in adult volunteers. We have now investigated the safety and immunogenicity of investigational Shigella sonnei and Shigella flexneri 2a conjugate vaccines in 1- to 4-year-old children, the age group at greatest risk for shigellosis.The O-specific polysaccharides of S. sonnei and S. flexneri 2a, the two most common shigellae from patients in Israel, were bound to medically useful carrier proteins to form conjugates. Eighty healthy 1- to 4-year-olds were randomized to receive two 0.5-ml im injections 6 weeks apart of either S. sonnei-CRM9 or S. flexneri 2a-r EPAsucc. Blood was taken before, 6 weeks after the first injection, 4 weeks after the second injections and 2 years after immunization for assay of IgG anti-lipopolysaccharide, diphtheria toxin and Pseudomonas aeruginosa exotoxin A antibodies by enzyme-linked immunosorbent assay.During an 8-day surveillance period after each immunization, low fever (37.8–39.0°C) lasting only 24 to 48 h occurred in 2 of 40 recipients after the first injection and 4 of 40 recipients after the second injection of S. flexneri 2a-r EPAsucc and in 2 of 38 of S. sonnei-CRM9 after the second injection; no fever was detected after the first injection. Liver function tests were normal in all vaccinees. S. sonnei-CRM9 elicited a >4-fold rise in IgG anti-LPS in 92.1% and S. flexneri 2a-r EPAsucc in 85% (P < 0.0001) after the second injection; both conjugates elicited type-specific booster responses. At 2 years the geometric mean concentrations of both IgG anti-lipopolysaccharides were significantly higher than preimmunization levels. A >4-fold rise of IgG anti-diphtheria (65.8%) and IgG anti-ETA (77.5%) was observed.These experimental Shigella conjugate vaccines were safe and immunogenic in 1- to 4-year-old children.