Vaccination with the CHAT strain of type 1 attenuated poliomyelitis virus in Leopoldville. Belgian Congo. 2. Studies of the safety and efficacy of vaccination.

In the course of the attenuated live poliovirus vaccination trial described in the preceding paper, an estimated 46 000 African children were given CHAT strain. Among the 3400 children followed up by home visits, none developed paralytic poliomyelitis or aspetic meningitis. When bled 2-3 months after vaccination, 60% of previously type-1 negative children had antibodies; interference by wild enteric viruses is suggested in explanation of this low figure.Two months after the beginning of vaccination, a type-1 poliomyelitis epidemic broke out, largely concentrated in a district in which no vaccination had yet been done. No geographical, chronological or family association between vaccination and poliomyelitis cases could be found. Of a total of 99 cases, 10 were in vaccinated children; but the intervals between vaccination and onset and identification of viruses from patients indicated that no case was caused by the vaccine.Previous vaccination with CHAT virus appeared to confer significant protection (60%) against the epidemic type 1 strain.