Fluctuating epidemiology of Poliovirus in conflict-affected regions: comparative trends in wild-type and vaccine-derived cases following vaccination campaigns

Suwanee, GA

Background

Poliomyelitis is a contagious enteroviral disease caused by poliovirus, which affects the central nervous system and causes irreversible paralysis. Two distinct forms of poliovirus exist today: wild poliovirus type 1 (WPV1), the naturally occurring virus associated with epidemics, and vaccine-derived poliovirus type 2 (cVDPV2), which arises when the weakened strain in the oral polio vaccine mutates and regains virulence in under-immunized communities. Since the start of the Global Polio Eradication Initiative (GPEI) in 1988, global incidence of WPV1 has declined due to widespread vaccination campaigns. However, sporadic cVDPV continues to occur, more often in conflict-affected regions where population displacement and weakened health infrastructure may allow vaccine-derived strains to circulate and occasionally cause disease.

Objective

To analyze trends in wild-type and vaccine-derived poliovirus incidence before and after mass vaccination campaigns in affected regions and to identify factors contributing to the persistence of vaccine-derived poliovirus transmission. While vaccination campaigns effectively suppress WPV1 transmission, incomplete and inefficient immunization in conflict-affected regions creates conditions that allow cVDPV to persist.

Methods

A systematic literature review with epidemiological data analysis was conducted using publicly available surveillance datasets from the World Health Organization (WHO), the GPEI, the Center of Disease Control and Prevention (CDC), and Our World in Data. Peer-reviewed data and surveillance reports were identified through searches of PubMed, Web of Science, Google Scholar, and ScienceDirect using keywords including “poliovirus”, “vaccine-derived poliovirus”, “vaccination campaigns”, “polio surveillance”, and “conflict-affected regions”. Studies published between 2017 and 2025 reporting poliovirus cases in conflict-affected regions were included. Twenty sources were found, and data were extracted on reported WPV1 and cVDPV cases before and after vaccination campaigns in Afghanistan, Pakistan, Gaza, Sudan, Yemen, Somalia, Syria, and DRC.

Results

Data analysis resulted in 548 cases of WPV1 and 2,194 cases of circulating cVDPV2. Persistent transmission of WPV1 was found in Pakistan and Afghanistan. In contrast, the remaining regions reported only cVDPV2 cases. Vaccine-derived outbreaks showed variability, for example, in several countries, cases declined following vaccination campaigns but rebounded in later years, suggesting incomplete interruption of transmission. For example, Yemen reported 31 cVDPV cases in 2020, increasing to 59 in 2021 and 160 in 2022 before declining to 8 cases in 2023 following vaccination campaigns; however, cases increased again to 187 in 2024. Similar patterns were observed in Sudan, Somalia, and the Democratic Republic of Congo. Additionally, environmental surveillance in regions detected poliovirus in wastewater, indicating ongoing silent transmission.

Discussion

These findings suggest the mass vaccination campaigns remain highly effective at reducing WPV1 transmission, even in conflict settings. Unlike WPV1, which requires an endemic reservoir, cVDPV outbreaks emerge in localized settings where population immunity is insufficient to limit transmission. Factors such as population displacement, sanitation breakdown, insecurity, and healthcare restriction may contribute. While supplementary immunization campaigns reduce infection rates, they may not consistently achieve sustained interruption of transmission in fragile settings. Strengthening routine immunization and surveillance, alongside the deployment of newer vaccines such as the genetically stabilized novel oral polio vaccine type 2, may be critical to advancing poliovirus eradication.