Effect of prior immunity on the shedding of virulent revertant virus in feces after oral immunization with live attenuated poliovirus vaccines

Groups of infants were immunized with one or two doses of orally inoculated live attenuated Sabin poliovirus vaccine (OPV group) or with one or two doses of enhanced-potency inactivated poliovirus vaccine (EIPV) administered parenterally followed by one or two doses of OPV (EIPV-OPV group). The fecal specimens from both groups were tested for poliovirus shedding 1-2 months after OPV. The virus isolates were examined for nucleic acid sequences in the 5' noncoding regions (bases 480,481, and 472 for serotypes 1,2, and 3, respectively) to determine whether the viruses shed represented nonattenuated revertants, attenuated parent vaccine strains (nonrevertants), or both. In the OPV group, 4 of the 6 virus isolates recovered 30-60 days after the first immunization dose and 1 of the 3 isolates obtained after the second dose were found to be nonrevertants (parent vaccine strain). In contrast, 11 of the 12 isolates in the EIPV-OPV group were of the nonvaccine revertant virus types. The frequency for reversion appeared to differ for different poliovirus serotypes. However, all revertant type 3 isolates were recovered from subjects previously immunized with EIPV.