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Polysaccharide-protein conjugate vaccines against invasive infections caused by Haemophilus influenzae type b (Hib) bacteria have been studied intensively in Finland since 1983. The vaccines were shown to be immunogenic in young infants and to induce immunologic memory. The protective efficacy against Hib meningitis and bacteremic infections was 87–95% after the primary immunization series in two consecutive efficacy trials. Hib conjugate vaccines also reduced oropharyngeal carriage of Hib bacteria among healthy children. Large scale vaccinations have led to elimination of Hib disease in Finland.