Co-Administration of Novel Oral Polio Vaccine Type 2 and Bivalent Oral Polio Vaccine: Current Evidence, Challenges, and Implications for the Polio Eradication Endgame

Abstract

Global polio eradication is challenged by persistent wild poliovirus type 1 (WPV1) transmission and outbreaks of circulating vaccine-derived poliovirus type 2 (cVDPV2). The 2016 global withdrawal of the type 2 component from trivalent oral poliovirus vaccine (tOPV) created population immunity gaps, facilitating cVDPV2 emergence in areas with low routine coverage. We conducted a narrative review synthesizing biological, immunological, clinical, epidemiological, and operational evidence on co-administration of novel oral poliovirus vaccine type 2 (nOPV2) with bivalent OPV (bOPV) as a strategy to rapidly expand population immunity during multi-serotype outbreaks. Co-administration provides potential logistical and equity advantages. Emerging randomized trial data, however, reveal significant immunological interference that can reduce type 2 immune responses. The effectiveness of this strategy is therefore context-dependent, with operational feasibility and coverage levels critically influencing outcomes. Co-administration of nOPV2 and bOPV should be applied as a context-specific tactical measure rather than a universal policy. Sustained high vaccination coverage and robust surveillance remain central to achieving global polio eradication.

Background

The Global Polio Eradication Initiative (GPEI), launched in 1988, has achieved extraordinary success, reducing global poliomyelitis incidence by over 99% and interrupting transmission of wild poliovirus (WPV) types 2 and 3.^1,2 Despite these achievements, the eradication of poliovirus remains incomplete. WPV1 continues to circulate endemically in limited geographic areas, while vaccine-derived polioviruses (VDPVs), particularly circulating vaccine-derived poliovirus type 2 (cVDPV2), have emerged as the dominant cause of paralytic polio worldwide.^2,3 The epidemiological shift toward cVDPV2 is directly linked to the 2016 globally synchronized switch from trivalent OPV (tOPV) to bivalent OPV (bOPV), which removed the Sabin type 2 strain from routine immunization schedules.⁴ While this switch was a carefully planned and necessary step to reduce the risk of type 2 vaccine-associated paralytic poliomyelitis and future VDPV emergence, it created large cohorts of children with little or no immunity to poliovirus type 2.^1,5 In settings characterized by low routine immunization coverage, fragile health systems, and poor sanitation, residual Sabin type 2 viruses continued to circulate, genetically evolve, and regain neurovirulence.⁶ Initial outbreak responses relied on monovalent OPV type 2 (mOPV2), but its use paradoxically carried the same evolutionary risks that gave rise to cVDPV2 in the first place.^7,8 This cycle necessitated the development of a more genetically stable vaccine.⁹ The novel oral poliovirus vaccine type 2 (nOPV2), designed through targeted genomic modifications, was therefore a pivotal innovation. Since its deployment under WHO Emergency Use Listing in 2021, nOPV2 has become the primary tool for cVDPV2 outbreak response.^6,10 Simultaneously, bOPV remains essential for maintaining immunity against WPV1 and preventing re-emergence of poliovirus types 1 and 3. Increasingly, countries face complex epidemiological realities in which multiple poliovirus serotypes co-circulate or pose imminent threats.^3,11 In such contexts, conducting multiple sequential vaccination campaigns is operationally challenging, costly, and often ineffective. This has led to growing interest in the co-administration of nOPV2 and bOPV as a strategy to efficiently address multiple immunity gaps in a single campaign round.¹² This narrative review examines the scientific rationale, available evidence, biological mechanisms, operational considerations, and strategic implications of nOPV2 and bOPV co-administration. Given the rapidly evolving evidence base and the absence of universal recommendations regarding simultaneous administration, this review aims to critically assess current knowledge, identify evidence gaps, and discuss the role of co-administration as a context-specific strategy within the global polio eradication endgame.

Methods

This narrative review was conducted to synthesize current evidence regarding the co-administration of novel oral poliovirus vaccine type 2 (nOPV2) and bivalent oral poliovirus vaccine (bOPV), with emphasis on biological mechanisms, immunogenicity, safety, epidemiological implications, and operational considerations for the polio eradication endgame. A literature search was conducted using PubMed, Scopus, Web of Science, Google Scholar, and official publications from the World Health Organization (WHO), Global Polio Eradication Initiative (GPEI), and Centers for Disease Control and Prevention (CDC). The search covered publications from January 2016 to March 2026, corresponding to the period following the global withdrawal of the type 2 component from trivalent oral poliovirus vaccine (tOPV). Search terms included combinations of: “novel oral poliovirus vaccine type 2”, “nOPV2”, “bivalent oral poliovirus vaccine”, “bOPV”, “co-administration”, “vaccine-derived poliovirus”, “circulating vaccine-derived poliovirus type 2”, “polio eradication”, “immunogenicity”, “mucosal immunity”, “oral poliovirus vaccine”, and “outbreak response”. Eligible sources included randomized controlled trials, observational studies, modelling analyses, surveillance reports, WHO technical documents, policy recommendations, and relevant narrative reviews published in English. Studies unrelated to nOPV2 or bOPV co-administration, and articles lacking relevant information on immunological, epidemiological, or operational outcomes were excluded. Evidence was narratively synthesized according to five major domains: (1) biological mechanisms and viral evolution, (2) immunogenicity and safety, (3) epidemiological and transmission implications, (4) operational feasibility, and (5) policy implications for the global polio eradication strategy.

Biological and Immunological Rationale for Co-Administration

Oral poliovirus vaccines confer protection through both systemic and mucosal immune mechanisms. Systemic humoral immunity, mediated by circulating neutralizing antibodies, protects individuals from paralytic disease.³ However, interruption of poliovirus transmission depends critically on intestinal mucosal immunity, which limits viral replication and shedding in the gut.¹³ This distinction is particularly important in low-income settings, where fecal–oral transmission predominates due to inadequate sanitation.¹⁴ Importantly, poliovirus immunity is serotype-specific, and immunity induced by one serotype provides little cross-protection against others. Historical data demonstrate that the Sabin type 2 strain replicates more efficiently in the intestinal tract and induces stronger mucosal immunity than types 1 and 3.¹⁴ While advantageous in isolation, this property has implications for viral competition and interference when multiple OPV strains are administered concurrently.¹⁴ Viral interference is a well-established phenomenon with live attenuated vaccines. During the tOPV era, the type 2 component was immunodominant and frequently suppressed immune responses to types 1 and 3.¹⁴ This necessitated careful balancing of viral titers within the vaccine formulation. Similar interference effects have been observed between OPV and other oral vaccines, such as rotavirus vaccines, particularly in high-enteric-burden settings.¹⁴ The co-administration of nOPV2 and bOPV therefore raises biologically plausible concerns that replication of one vaccine virus may inhibit the intestinal “take” of another. Whether such interference would be reciprocal or asymmetric, and whether it would compromise population-level immunity, were key questions that required empirical evaluation.^3,14 Despite these immunological concerns, the potential benefits of co-administration are substantial: Accelerated closure of multiple immunity gaps in a single campaign round; reduced operational complexity, including fewer campaigns and lower costs; improved equity, particularly in insecure or inaccessible settings; minimized campaign fatigue among communities and frontline health workers. These advantages are especially compelling in emergency contexts where opportunities to vaccinate are infrequent and unpredictable.^15,16

Evolution of Vaccine-Derived Polioviruses and Genetic Stability of OPV Strains

The emergence of vaccine-derived polioviruses (VDPVs) represents a complex interaction between vaccine biology, population immunity, and viral evolution. Oral poliovirus vaccines contain live attenuated strains capable of replication in the intestinal tract.¹⁷ In populations with insufficient immunity, prolonged circulation of vaccine-derived viruses may allow accumulation of genetic mutations that partially restore neurovirulence and transmissibility. Following the global withdrawal of the type 2 component from trivalent OPV in 2016, reduced population immunity against poliovirus type 2 created ecological conditions that facilitated the emergence and spread of circulating vaccine-derived poliovirus type 2 (cVDPV2).¹⁸ The persistence of type 2 vaccine-related viruses in under-immunized populations demonstrated the importance of maintaining high vaccination coverage and strong surveillance systems.¹⁸ The development of nOPV2 aimed to address these challenges through targeted genetic modifications designed to improve genomic stability while maintaining immunogenicity.¹⁹ Compared with the Sabin type 2 strain used in monovalent OPV type 2 (mOPV2), nOPV2 contains engineered modifications that reduce the probability of reversion toward neurovirulent phenotypes.²⁰ However, genetic stability does not eliminate the need for continuous surveillance. Viral evolution remains influenced by replication dynamics, immune pressure, population immunity gaps, and ecological conditions.²¹ Therefore, genomic sequencing and environmental surveillance remain essential components of outbreak response strategies involving nOPV2.

Viral Competition, Selection Pressure, and Immunological Interference

The intestinal replication environment plays a central role in determining OPV effectiveness. Following oral administration, vaccine strains replicate in the gut and induce mucosal immunity that contributes to interruption of poliovirus transmission.²² When multiple live attenuated poliovirus strains are administered simultaneously, competition may occur during intestinal replication. Differences in replication fitness, viral load, and timing of replication may influence vaccine take and immune response.²³ Historical experience with trivalent OPV demonstrated that the type 2 Sabin strain could dominate intestinal replication and suppress responses against other serotypes. Similar biological mechanisms may explain the reduced type 2 immune response observed when nOPV2 is administered concurrently with bOPV.²⁴ The available evidence suggests that interference is primarily asymmetric, with nOPV2 responses being more affected than responses to poliovirus types 1 and 3.²⁵ However, the magnitude and public health significance of this interference depend on several factors, including baseline immunity, vaccination coverage, transmission intensity, nutritional status, intestinal microbiota, and operational quality of vaccination campaigns.

Evidence on Immunogenicity

The most robust evidence on immunogenicity comes from a 2023 open-label, randomized, non-inferiority trial conducted in Bangladesh among 736 OPV-naïve infants. This study provides the first direct, controlled comparison of immune responses following co-administration versus separate administration of nOPV2 and bOPV.²⁶ The trial demonstrated a substantial reduction in type 2 seroconversion when nOPV2 was co-administered with bOPV, with immune response rates declining from 86% in the nOPV2-only group to 65% in the co-administration group.²⁶ This difference exceeded the predefined non-inferiority margin and represents a clinically meaningful loss of protection against the serotype currently responsible for the majority of outbreaks. In contrast, immune responses to poliovirus types 1 and 3 were preserved, indicating that interference was asymmetric and predominantly affected the nOPV2 component.³ Reduced shedding of type 2 virus in stool samples following co-administration further supports the hypothesis that bOPV strains interfere with nOPV2 replication in the gut.²⁷ These findings fundamentally reshape the risk–benefit calculus of co-administration and underscore the need for careful contextual decision-making.

Safety and Genetic Stability

Both nOPV2 and bOPV have demonstrated excellent safety profiles across clinical trials and large-scale field use.²⁸ Serious adverse events have been rare and not causally linked to vaccination. The defining safety advantage of nOPV2 lies in its enhanced genetic stability, achieved through targeted modifications that reduce the likelihood of reversion to neurovirulence.^19,20 There is currently no evidence that co-administration with bOPV compromises this stability. While theoretical concerns regarding genetic recombination between vaccine strains exist, extensive genomic surveillance following the administration of hundreds of millions of nOPV2 doses has not identified this as a practical risk.^21,22 Continued molecular surveillance remains essential, particularly as vaccination strategies evolve.

Epidemiological and Transmission Implications

The epidemiological justification for co-administration is strongest in areas facing simultaneous or sequential outbreaks involving multiple serotypes, or where surveillance indicates imminent vulnerability. In such settings, the speed of delivering broad immunity may outweigh reduced per-dose efficacy against type 2.²⁹ From a population perspective, eradication is fundamentally a coverage-driven endeavor. Lower individual immunogenicity may be offset if co-administration substantially increases the proportion of children reached. As has been repeatedly emphasized in the polio endgame, “coverage trumps efficacy” when transmission is intense.^7,24,25 Preliminary modeling suggests that in high-risk, low-coverage environments, co-administration could outperform sequential campaigns.¹⁶ However, these models must now integrate empirical immunogenicity data demonstrating interference. Real-world observational data remain limited but will be critical for refining strategic guidance.³⁰

Role of IPV in nOPV2–bOPV Co-Administration Strategies

Inactivated poliovirus vaccine (IPV) has become increasingly important in the post-switch era because of its ability to induce strong systemic immunity against all three poliovirus serotypes without the risk of vaccine-derived virus emergence. Although IPV provides limited intestinal immunity compared with OPV, IPV priming may enhance protection against paralytic disease and contribute to population immunity when combined with OPV-based strategies. Sequential schedules incorporating IPV and OPV may therefore represent an important approach for countries balancing outbreak response needs with long-term eradication objectives.³¹ The potential interaction between IPV priming and nOPV2–bOPV co-administration requires further investigation.³² Future studies should evaluate whether IPV administration before or alongside OPV improves immune responses, reduces viral shedding, and influences transmission interruption.

Programmatic and Operational Considerations

Co-administration introduces additional logistical complexity, including increased cold-chain requirements, dual-vial management, and enhanced training needs for vaccinators.^27,28 Clear labeling, robust micro-planning, and strong community communication are essential to prevent errors and maintain trust. Policy endorsement would require formal review by WHO SAGE and national advisory bodies.^33,34 In conflict-affected and hard-to-reach areas, co-administration may offer a pragmatic means of maximizing protection during rare vaccination opportunities. In such contexts, partial immunity to multiple serotypes is often preferable to no immunity at all, making equity considerations central to decision-making.^15,35 Hoewer; Critical gaps remain, including: Optimal dosing schedules to mitigate interference;direct transmission and shedding studies; enhanced post-campaign environmental surveillance; integration of IPV priming strategies. Addressing these gaps is essential for evidence-based policy refinement.^29,36 Nevertheless; Co-administration should be viewed as a targeted tactical option, not a universal solution. Its use must be guided by epidemiology, operational feasibility, and equity considerations. Ultimately, the success of polio eradication depends less on vaccine configuration than on the ability to consistently reach every child.^34,37

Conclusion

The co-administration of nOPV2 and bOPV represents a promising but context-dependent strategy for addressing multiple poliovirus immunity gaps during complex outbreak situations. Available evidence demonstrates important operational advantages, including improved campaign efficiency, reduced logistical burden, and increased opportunities to reach underserved populations. However, emerging clinical evidence indicates that concurrent administration may reduce type 2 immune responses through intestinal viral interference. Although nOPV2 provides improved genetic stability compared with earlier OPV formulations, continued genomic surveillance remains essential to monitor viral evolution and ensure safe deployment.³⁸ Current evidence remains limited, and further research is required to determine the effectiveness of co-administration under real-world conditions. Priority areas include field effectiveness studies, transmission analyses, optimal dosing strategies, integration with IPV-based schedules, and operational research in low-resource and conflict-affected settings.³⁹ Ultimately, the success of the polio eradication endgame will depend not only on vaccine innovation but also on achieving and maintaining high immunization coverage, strengthening surveillance systems, and ensuring equitable access to vaccination services.

Funding Statement

No funding was received for this work.

Data Sharing Statement

Not applicable. This manuscript is based on published literature and publicly available reports.

Author Contributions

All authors made a significant contribution to the work reported, whether that is in the conception, study design, execution, acquisition of data, analysis and interpretation, or in all these areas; took part in drafting, revising or critically reviewing the article; gave final approval of the version to be published; have agreed on the journal to which the article has been submitted; and agree to be accountable for all aspects of the work.

Disclosure

The authors declare that they have no competing interests.