As the Global Polio Eradication Initiative (GPEI) strategizes towards the final steps of eradication, routine immunization schedules evolve, and high-quality vaccination campaigns and surveillance systems remain essential. New tools are consistently being developed, such as the novel oral poliovirus vaccine to combat outbreaks more sustainably, as well as non-infectiously manufactured vaccines such as virus-like particle vaccines to eliminate the risk of resurgence of polio on the eve of a polio-free world. As the GPEI inches towards eradication, re-strategizing in the face of evolving challenges and preparing for unknown risks in the post-certification era are critical.

In June 2018, a type 1 circulating vaccine-derived poliovirus (cVDPV1) outbreak was declared in Papua New Guinea (PNG), resulting in a total of 26 paralytic confirmed cases. Eight vaccination campaign rounds with bivalent oral poliovirus vaccine (bOPV) were carried out in response. Prevalence of neutralizing polio antibodies in children was assessed two years after the outbreak response was completed. We conducted a cross-sectional serological survey among children aged 6 months-10 years selected from six provinces in PNG to evaluate seroprevalence of neutralizing polio antibodies to the three poliovirus serotypes and analyse sociodemographic risk factors. We included 984 of 1006 enrolled children in the final analysis. The seroprevalence of neutralizing polio antibodies for serotype 1, 2 and 3 was 98.3% (95% CI: 97.4-98.9), 63.1% (95% CI: 60.1-66.1) and 95.0% (95% CI: 93.6-96.3), respectively. Children <1 year had significantly lower type 1 seroprevalence compared to older children (p < 0.001); there were no significant differences in seroprevalence among provinces. PNG successfully interrupted transmission of cVDPV1 with several high coverage bOPV campaigns and seroprevalence remained high after two years. The emergence of cVDPV strains underscores the importance of maintaining high levels of routine immunization coverage and effective surveillance systems for early detection. World Health Organization through a Rotary International IPPC grant.

The development of fractional-dose inactivated poliovirus vaccine (fIPV, a fifth of a full IPV dose) has provided a dose-stretching, less costly, and immunogenic alternative to full dose IPV, enhancing global IPV introduction and accessibility. fIPV is administered intradermally, however, intradermal injection is considered difficult and requires skilled administration. This study aimed at assessing the non-inferiority of intramuscular versus intradermal administration of fIPV in young infants in Maputo, Mozambique. This was an open label, randomized, non-inferiority trial conducted between 2020 and 2022 (Ref. NCT04027036). Healthy IPV-naive infants attending routine immunization services were enrolled in the study and block randomized to receive two sequential doses of fIPV (0.1 mL), either intramuscularly or intradermally, at two and four months of age. Blood samples were collected at four timepoints: baseline, two months after the first dose, one week after the second dose, and one month after the second dose. Samples were analyzed via microneutralization assays against all three poliovirus serotypes. The primary endpoint was defined as cumulative seroconversion rates to type 2 poliovirus after administration of two doses of intramuscular versus intradermal fIPV. The study has been completed, and data analyzed per protocol. 382 infants (49.5% [189/382] female) were enrolled in the study between 12 August 2020 and 31 May 2022. Cumulative type 2 seroconversion after two fIPV doses reached 92.1% (139/151, 95% CI 86.5–95.8) in the intradermal group and 96.1% (148/154, 95% CI 91.7–98.6) in the intramuscular group (=0.15). The most common adverse events were upper respiratory tract infections, not related to the study vaccine. 12 serious adverse events occurred in the study, none were considered to be related to the study vaccines. This is the second study demonstrating non-inferiority of fIPV administered intramuscularly compared with intradermally; and the first study assessing immunogenicity in young infants. The results informed vaccine policymaking, leading to a recent recommendation from the Strategic Advisory Group of Experts on Immunization from WHO on intramuscular administration of fIPV in outbreak response. World Health Organization.