Different Emergence Groups of Circulating Vaccine-Derived Poliovirus Type 2 (cVDPV2-n) Outbreak Linked to Novel Oral Polio Vaccine Type 2 in a Chronic Humanitarian Contexts. A Retrospective Descriptive Analysis, South Sudan, 2023–2024

Background: South Sudan detected an outbreak of circulating vaccine derived poliovirus linked to the novel Oral Polio Vaccine, previously not in use in the country. This study retrospectively examines the response, highlighting best practices, challenges and lessons learnt for posterity. Methods : A retrospective descriptive epidemiological study utilizing multiple data sources, including surveillance, immunization records, and field investigation reports. Result: South Sudan reported its first documented outbreak of circulating Vaccine-Derived Poliovirus Type 2 (cVDPV2-n) on December 14, 2023, with initial cases detected in Yambio County, Western Equatoria State, and later in Juba, Central Equatoria State. The index case was an unvaccinated 19-month-old boy, who developed paralysis on October 31, 2023. Epidemiological investigations linked to population movements from neighboring countries of DRC, CAR, and Burundi, which previously had used nOPV2 in responding to polio outbreaks. Laboratory-backed surveillance documented four distinct emergency groups and ambiguous VPDV2, affecting 13 counties across five states. Each group exhibited unique genetic and geographic traits, with nucleotide variations ranging from 6 to 30 changes. Overall, three paralytic infections of cVDPV2 were reported in 2023, and ten in 2024. An additional 9 VDPV2 isolates were detected of which five of them were from environmental samples. In 2024 alone, 18 VDPV2 isolates were confirmed from environmental surveillance samples, classified as circulating (13) and ambiguous (5). The outbreak response began with the reactivation of the Emergency Operations Center (EOC) and appointment of incident managers, which laid the ground for effective coordination. A risk assessment feeding into a response plan was completed by November 14th, 2023. Contrary to the polio outbreak response SOPs, the initial zero-dose round was not conducted within 14 days post-confirmation, because of delayed approval for vaccines and funding which was secured one month later. The first two rounds in February and April 2024 achieved administrative coverage rates of 103% and 111%, respectively. However, breakthrough cases were detected in epidemiological week 35 and two additional rounds were approved and implemented in November 2024 and January 2025 with coverage of 103% and 106% of the 3.4 million target respectively. Conclusion : The cVDPV2-n outbreak involved three distinct emergency groups underscore the challenges faced with low immunization coverage, in fragile and vulnerable countries. It emphasizes the urgent need to provide for innovative financing solutions for routine vaccination, flexible preventive vaccination policies, cross-border coordination and multi-country risk assessments to inform vaccine deployments for prevention importations.