Enhanced Immunisation Crucial for Eradicating Variant Poliovirus Globally

A leading US-based public health expert has underscored the critical role of improved immunisation in the ongoing global fight against polio, specifically in tackling the emergence of variant poliovirus strains. Dr. Ananda Sankar Bandyopadhyay, Deputy Director at the Gates Foundation, highlighted that while oral polio vaccines (OPVs) are foundational to global eradication efforts due to their safety, effectiveness, affordability, and ease of delivery, there is a pressing need for enhanced immunisation strategies to combat vaccine-derived poliovirus strains. These variant strains, known as circulating vaccine-derived polioviruses (cVDPVs), primarily emerge and spread in communities where immunisation coverage is persistently low. Polio, a highly infectious disease caused by a virus that invades the nervous system, has no cure, but it is entirely preventable through vaccination. The Global Polio Eradication Initiative (GPEI), established in 1988, has made remarkable progress, reducing polio cases by over 99% worldwide and preventing an estimated 20 million cases of paralysis in children. Wild poliovirus type 2 (WPV2) and type 3 (WPV3) have been declared eradicated, with only wild poliovirus type 1 (WPV1) remaining endemic in just two countries: Afghanistan and Pakistan. However, the challenge of cVDPVs has grown, becoming a major obstacle to complete eradication. cVDPVs originate from the live, weakened poliovirus contained in OPV. When this vaccine virus replicates in the human gut, it can genetically change. In populations with insufficient vaccination, especially in areas with poor hygiene, sanitation, or overcrowding, these altered strains can circulate for extended periods, mutate further, and eventually regain the ability to cause paralysis, much like wild poliovirus. Low immunization rates are consistently identified as the key risk factor for the emergence and spread of cVDPVs. The polio vaccine, whether Inactivated Poliovirus Vaccine (IPV) or Oral Polio Vaccine (OPV), offers protection against both wild poliovirus and vaccine-derived poliovirus. India stands as a significant success story in polio eradication. The country was certified polio-free in March 2014, having reported its last case of wild poliovirus in January 2011. Dr. Bandyopadhyay noted that paralytic outbreaks from circulating variant polioviruses have not been detected in India in recent years, which he attributes to the country's ability to maintain high routine immunisation coverage. India's routine immunisation coverage has climbed to over 93 percent, supported by robust campaigns like the Pulse Polio Immunization Programme launched in 1995, and the Universal Immunization Programme (UIP). The infrastructure developed for polio eradication, including high-quality disease surveillance and contact tracing, has become a backbone for India's broader health goals. Despite this success, health experts caution against complacency, as variant poliovirus outbreaks continue to emerge globally in areas with low immunisation coverage. To counter this, significant advancements in vaccine technology are being deployed. The novel oral polio vaccine type 2 (nOPV2), released in 2021 under the WHO's Emergency Use Listing procedure and later prequalified, is a key tool in controlling these variant outbreaks. nOPV2 is more genetically stable and less likely to revert to a virulent form compared to older OPV versions, thereby reducing the risk of cVDPV2 outbreaks. Approximately 1.3 billion doses of nOPV2 have been administered in 41 countries during nearly 250 outbreak response campaigns since 2021, contributing to a marked decline in new variant polio outbreaks. Hyderabad-based Biological E is notably one of only two manufacturers globally producing nOPV2. Furthermore, the optimal use of Inactivated Poliovirus Vaccine (IPV) alongside OPVs is crucial for closing immunity gaps and minimising the risks associated with variant strains. The WHO recommends the introduction of at least one dose of IPV into all routine immunisation programmes globally. IPV protects individuals against all three types of poliovirus and cannot cause the disease, as it contains killed virus. While IPV protects individuals, it does not stop the transmission of the virus in the gut as effectively as OPV. India introduced IPV in 2015 as part of its commitment to the Global Polio Endgame Strategy. The Global Polio Eradication Initiative's Polio Oversight Board extended the timelines for eradication to the end of 2027 for wild poliovirus and the end of 2029 for type 2 variant poliovirus, acknowledging ongoing challenges such as conflicts, climate emergencies, and persistent insecurity in certain regions that hinder vaccine delivery. The collective efforts of organisations like the WHO, UNICEF, and the Gates Foundation, alongside governments and partners, are essential to maintain high vaccination rates and prevent the reintroduction of the virus, safeguarding future generations globally. The overarching message from experts and global health bodies remains clear: sustained, high-quality immunisation coverage is the most effective defense against all forms of poliovirus, including the challenging variant strains.

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