Vaccination with the CHAT strain of type 1 attenuated poliomyelitis virus in Leopoldville, Congo. 3. Safety and efficacy during the first 21 months of study.

Between August 1958 and April 1960 nearly all children less than five years old in Léopoldville were orally vaccinated with the CHAT, type 1, attenuated poliovirus strain of Koprowski. In a previous report, the first nine months of the campaign were reviewed and tentative conclusions were drawn concerning the safety and efficacy of the vaccine. The present paper is a report on a further twelve-month period of observation. Additional epidemiological data and virological studies, including the use of virus markers for assessing the safety of the attenuated strain, lend added support to the earlier conclusions. Of particular interest were the results of virus isolations from patients with paralytic poliomyelitis, there being an apparent shift in the predominant causative agent from type 1 to type 3 poliovirus concomitant with the completion of the type 1 vaccination campaign.