Clinical Trials in Infants of Orally Administered Attenuated Poliomyelitis Viruses

Forty-six infants, ranging from less than 1 day to 6 months of age, were given more than 100 feedings of living, attenuated poliomyelitis viruses without the occurrence of major or minor illness. The strains used were CHAT (type 1), Wistar (type 1), Jackson (type 2), P-712 (type 2) and Fox (type 3). All strains except the Jackson strain were found to be antigenic on oral administration. Response to vaccination was demonstrated in these infants by the presence after vaccination of antibody levels significantly in excess of those attributable to transplacentally acquired antibodies, and by the detection of fecal excretion of poliomyelitis virus. Infants less than 2 months old were more difficult to immunize than older infants. The evidence suggests that biologic immaturity rather than transplacental antibodies caused the difference. When the three types of poliomyelitis virus were fed at 3-week intervals, responses occurred to all types. No interference between types was observed when they were fed in all possible sequences. Three infants given a second feeding of homotypic, attenuated poliomyelitis virus 3 to 5 months after a successful vaccination showed resistance to intestinal reinfection.