In 1973, a type 1 poliomyelitis epidemic in Kenya was curtailed at an early stage by two mass distributions of trivalent oral vaccine. It was considered useful to know the immunity status of the child population that had resulted from the vaccine distributions and that had presumably contributed to its control. We also wished to know to what extent wild and vaccine virus strains were in circulation after the mass vaccination campaign. Anal swabs and blood were collected from a sample of the children in four areas where the efficiency of vaccine distribution had varied, and the results of virus isolation attempts and antibody tests are reported. Three poliovirus strains were isolated. It was surprising that, in general, the herd immunity after two vaccination rounds did not substantially differ from that found in Kenya on other occasions. Possible reasons for these results are discussed.
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