Comparative evaluation of immunization with live attenuated and inactivated poliovirus vaccines

The development of serum and nasopharyngeal antibody response, as well as the magnitude and temporal pattern of fecal shedding of vaccine and revertant polio-viruses, have been examined in infants previously immunized with one or more doses of orally administered live attenuated poliovaccine, enhanced potency inactivated polio-vaccine, or both. The nature of serum immune response appears to be similar after either immunization schedule, although the antibody titers are quantitatively higher after two doses of EP-IPV than those observed after a similar schedule with OPV. Highest antibody activity is generally detected in subjects immunized with a combination of EP-IPV followed by OPV. ELISA antibody activity in the nasopharynx was regularly detected after either form of immunization. However, neutralizing and VP3 poliovirus virion protein-specific antibody responses in the nasopharynx were consistently observed in subjects immunized with OPV or EP-IPV followed by OPV. Subjects immunized with EP-IPV alone exhibit significantly lower or absent neutralizing or VP3-specific responses. The nucleic acid sequences of the purified RNA obtained from all virus isolates have also been examined in the 5' noncoding region by dideoxy-sequencing to determine whether the viruses shed represent revertants (vaccine), non-revertants, or both. The frequency and duration of vaccine virus shedding appears to be similar in both immunization schedules. Revertant virus shedding was not demonstrated 30 days after immunization with OPV alone. However, shedding of revertants was detected for as long as 60 days in some subjects previously immunized with EP-IPV. The duration of shedding of revertant virus differed with different serotypes and different immunization regimens. Prior immunization with one or more doses of OPV reduced the length of shedding of revertant virus. Significantly, however, prior immunization with one or more doses of EP-IPV was not associated with reduced shedding of revertant virus types. Based on these observations and a number of other epidemiologic data summarized in this review, it is clear that both OPV and EP-IPV when used alone are highly effective and safe in inducing effective immunity to polio-virus and in the eradication of poliomyelitis. While the combination schedule employing EP-IPV followed by OPV should result in a decline of vaccine-associated paralytic (VAP) disease in OPV recipients, such immunization schedules may have little or no impact on the development of VAP in susceptible contacts. Furthermore, the logistics and the cost of combination schedules must be considered before current recommendations based on the use of OPV or EP-IPV alone are revised.