Comparison of Immunogenicity and Safety of Sabin and Salk Inactivated Poliovirus Vaccine in Infant Immunization

Poliomyelitis was the leading cause of permanent disability before highly effective oral poliovirus vaccine (OPV) and Salk Inactivated Poliovirus vaccine (IPV) became available. Lately polio disease is largely associated with vaccine strains in oral poliovirus vaccine (OPV), Vaccine Associated Paralytic Poliomyelitis (VAPP) outlining the need to introduce strategies to mitigate the risk of VAPP while maintaining population immunity to polioviruses. In 2015, Strategic Advisory Group of Experts (SAGE) recommended staged OPV withdrawal starting with trivalent (tOPV) to bivalent OPV (bOPV) switch and addition of single dose IPV in OPV using countries but supply constraints resulted in delayed implementation. In 2020, SAGE recommended adding another dose of IPV in routine immunization to maintain population immunity to type 2 and reduce continual threat of polio re-emergence. However, conventional Salk-IPV is expensive with limited supply due to high-quality containment requirements of manufacturing. Sabin IPV (sIPV), a trivalent inactivated poliovirus preparation developed from Sabin strains of oral polioviruses type 1, 2 and 3 has shown good immunogenicity against all poliovirus types. Sabin IPV has been shown non-inferior to Salk IPV in head-to-head trials opening feasibility to use this vaccine in national programs. Immunogenicity data on sequential bOPV-Salk IPV with one or two doses of Salk-IPV (both full dose and fractional) are available but similar data is lacking for bOPV-Sabin IPV sequential schedules. Data on bOPV-Sabin IPV sequential schedules may be particularly important for LMIC countries like Nepal that use fIPV schedules for cost and resources management. We intend to generate more evidence on bOPV-Sabin IPV sequential schedule so that Sabin IPV can become a choice or an alternative to Salk IPV in national immunization schedules. We aim to conduct a double blind, non-inferiority, randomized controlled trial to evaluate immunogenicity of sequential bOPV + 2 dose Sabin IPV polio immunization schedule. Healthy Nepali infants attending immunization clinics (230 infants in each arm) for their first dose of pentavalent and polio vaccines will be enrolled, randomized to one of the two study arms and followed up till the end of study at 10 months. Routine immunization will be administered to all infants along study intervention of conventional or Sabin IPV at 14 weeks and 9 months. Blood samples will be obtained from infants at 6 weeks, 18 weeks and 10 months. At the end of study, we will compare proportion of children seropositive against type 2 poliovirus following two doses of Salk IPV versus Sabin IPV. We believe that this study will add evidence on comparability of immunogenicity and safety of 2 doses of Sabin IPV over conventional Salk IPV in sequential schedule following bOPV in routine immunization.