Describe the challenges of circulating Vaccine-Derived Poliovirus (VDPV). Add a note on the environmental surveillance and preventive strategies for effective maintenance of Polio elimination.

The global fight against polio has reached its endgame, with wild poliovirus transmission at an all-time low, primarily confined to Afghanistan and Pakistan. Nevertheless, the emergence of Vaccine-Derived Poliovirus (VDPV) continues to pose a significant threat to global polio eradication efforts. These variants, originating from the live attenuated oral polio vaccine (OPV), can regain neurovirulence and transmissibility in communities with low immunization coverage, leading to outbreaks of paralytic polio.

Challenges of Circulating Vaccine-Derived Poliovirus (cVDPV)

The circulation of VDPVs presents multifaceted challenges for public health:

• Risk of Paralysis and Outbreaks: While rare, cVDPVs can cause paralytic polio clinically indistinguishable from wild poliovirus (WPV). These outbreaks can undermine confidence in vaccination programs and set back eradication efforts. For instance, during January 2023–June 2024, 74 cVDPV outbreaks were detected in 39 countries, resulting in 672 confirmed acute flaccid paralysis (AFP) cases in 27 countries.
• Maintaining Polio-Free Status: Countries declared polio-free, like India (since 2014), face the constant threat of re-infection or detection of VDPVs, which, while not WPV, necessitate immediate public health responses. For example, a vaccine-derived polio case was confirmed in Meghalaya, India, in August 2024, prompting high alert despite India's polio-free status.
• Genetic Instability of OPV: The live-attenuated virus in OPV can replicate in the gut, especially in immunocompromised individuals or in areas with low population immunity, allowing for genetic mutations over time. This reversion to neurovirulence and increased transmissibility is the root cause of VDPVs.
• Difficulty in Differentiation: Differentiating between wild poliovirus and VDPV requires sophisticated laboratory testing, which can delay outbreak response and complicate surveillance efforts.
• Low Immunization Coverage: VDPV outbreaks are directly linked to community-level immunity gaps. Weak routine immunization programs, poor quality supplementary immunization activities, insecurity, civil conflict, and vaccine hesitancy contribute to a high percentage of undervaccinated children, allowing the virus to circulate unchecked.
• International Spread: cVDPVs can spread across borders, especially from regions with persistent transmission like Nigeria and Somalia to neighboring countries, as observed between January 2023 and June 2024. This necessitates robust cross-border coordination.
• Vaccine Transition Complexity: The global transition from trivalent OPV (tOPV) to bivalent OPV (bOPV) and ultimately to inactivated poliovirus vaccine (IPV) aims to eliminate the risk of VDPV types 2 and 3, but managing this transition effectively while maintaining high immunity is complex.

Environmental Surveillance for Polio Elimination

Environmental surveillance (ES) plays a critical complementary role to acute flaccid paralysis (AFP) surveillance in maintaining polio elimination. It involves systematically testing sewage or other environmental samples for the presence of poliovirus.

Methods of Environmental Surveillance:

• Sampling Sites: Samples are typically collected from wastewater treatment plants or open sewage systems, particularly in high-risk urban areas or populations with suspected deficiencies in AFP surveillance. The preferable size of the source population for a sampling site is 100,000–300,000.
• Sampling Frequency: Regular sampling, ideally twice a month but at least once a month, is crucial to provide timely detection of poliovirus circulation.
• Laboratory Analysis: Collected samples undergo laboratory methods for concentration, separation, and identification of poliovirus. Genetic sequencing is then performed to characterize the isolated viruses (wild, vaccine-derived, or vaccine-like) and determine their genetic relationship to known strains, helping to map transmission paths. India uses a precipitation method for virus isolation from samples.
• Detection of Asymptomatic Circulation: A key advantage of ES is its ability to detect poliovirus shed in the feces of infected individuals, whether symptomatic or asymptomatic. This provides an early warning system for outbreaks, often before clinical cases of paralysis appear.
• Monitoring Population Immunity: ES can assess population immunity, particularly in areas vaccinated with IPV, which prevents paralysis but not necessarily infection and shedding.
• Evaluating Outbreak Response: ES also helps in assessing the quality and effectiveness of vaccination campaigns, as it can detect the vaccine virus used in immunization activities.

Preventive Strategies for Effective Maintenance of Polio Elimination

A multi-pronged approach involving robust immunization, rapid response, and continuous surveillance is essential:

• High and Equitable Immunization Coverage: The cornerstone of polio elimination is achieving and sustaining high routine immunization coverage with either OPV or Inactivated Polio Vaccine (IPV) in all populations. This creates sufficient herd immunity to stop poliovirus circulation.
• Rapid Outbreak Response: Prompt and high-quality supplementary immunization activities (SIAs) using appropriate vaccines (e.g., novel oral polio vaccine type 2 - nOPV2 for cVDPV2 outbreaks) are vital in areas where cVDPV or WPV is detected. Timely allocation of resources and effective implementation are crucial to prevent further spread.
• Enhanced Surveillance: Strengthening both AFP surveillance (the "gold standard" for detecting paralytic polio cases) and environmental surveillance is critical. This includes improving reporting completeness and timeliness, expanding ES networks, and ensuring laboratories have the capacity for rapid diagnosis and genetic sequencing. The Global Polio Surveillance Action Plan 2025-2026 emphasizes this.
• Phased Withdrawal of OPV: The Global Polio Eradication Initiative (GPEI) strategy involves the gradual phasing out of OPV globally in favor of IPV to eliminate the risk of VDPVs. The switch from tOPV to bOPV in 2016 was a major step in this direction.
• Addressing Underserved Populations: Special efforts are needed to reach children in security-compromised areas, mobile populations, and communities with vaccine hesitancy due to misinformation. This requires innovative strategies, community engagement, and cross-border coordination.
• Development of New Vaccines: The novel oral polio vaccine type 2 (nOPV2), launched in March 2021, is designed to be more genetically stable and less prone to reverting to virulence, making it a crucial tool for stopping cVDPV2 outbreaks more sustainably.
• Strengthening Primary Healthcare: Integrating polio vaccination into broader primary healthcare systems and strengthening routine immunization infrastructure helps ensure sustained coverage and resilience against outbreaks.

Aspect	Wild Poliovirus (WPV)	Vaccine-Derived Poliovirus (VDPV)
Origin	Naturally occurring virus	Mutated strain from live-attenuated OPV
Causative Factor	Direct infection from person-to-person	Prolonged circulation of OPV strain in under-immunized populations
Paralysis Risk	High, approximately 1 in 200 infections	Can cause paralysis, similar to WPV, when mutated
Current Endemicity	Afghanistan, Pakistan	Outbreaks in multiple countries, primarily Africa (as of 2023-2024)
Prevention	High immunization with OPV or IPV	High immunization with OPV or IPV; phased withdrawal of OPV