COMBINED IMMUNIZATION OF INFANTS WITH ORAL AND INACTIVATED POLIOVIRUSVACCINES - RESULTS OF A RANDOMIZED TRIAL IN THE GAMBIA, OMAN, AND THAILAND

To assess an immunization schedule combining oral (OPV) and inactivate d poliovirus vaccines (IPV), we conducted a clinical trial in the Gamb ia, Oman, and Thailand. Children were randomized to receive one of the following schedules: OPV at birth, 6, 10, and 14 weeks of age; OPV at birth followed by both OPV and IPV at 6, 10, and 14 weeks of age; or placebo at birth followed by IPV at 6, 10, and 14 weeks of age. A tota l of 1685 infants were enrolled; 24-week serum specimens were availabl e for 1291 infants (77%). Across the study sites at 24 weeks of age, t he proportion of seropositive children in the combined schedule group was 95-99% for type 1, 99-100% for type 2, and 97-100% for type 3. In the Gambia and Oman, the combined schedule performed significantly bet ter than OPV for type 1 (95-97% versus 88-90%) and type 3 (97-99% vers us 72-73%). In the Gambia and Oman, seroprevalences in the IPV group w ere lower for type 1 (significantly lower in the Gambia); significantl y lower for type 2; and significantly higher for type 3, compared with the OPV group. In Thailand, the IPV group had significantly lower pro portions of children who were seropositive for each of the three types , compared with the OPV group. The responses to OPV in the Gambia, Oma n, and Thailand were consistent with previous studies from these count ries. IPV given at 6, 10, and 14 weeks of age provided inadequate sero logical protection against poliovirus, especially type 1. The combined schedule provided the highest levels of serum antibody response, with mucosal immunity equivalent to that produced by OPV alone.