Vaccine derived polio virus
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Vaccine-Derived Poliovirus (VDPV)
Background
Poliovirus is a member of the Picornaviridae family (genus Enterovirus). Two vaccines exist:
- Inactivated Polio Vaccine (IPV) — Salk; injectable; no live virus
- Oral Polio Vaccine (OPV) — Sabin; live attenuated; the source of VDPV
VDPV arises when the live attenuated poliovirus in OPV mutates during prolonged replication — in an individual host or via person-to-person transmission — and regains neurovirulence and transmissibility approaching that of wild-type poliovirus. — Harrison's Principles of Internal Medicine, 22E (2025)
Types of VDPV
| Type | Abbreviation | Mechanism |
|---|---|---|
| Circulating VDPV | cVDPV | Spreads in under-immunized communities; most common form causing outbreaks |
| Immunodeficiency-associated VDPV | iVDPV | Shed for years (>25 years reported) by immunocompromised individuals (e.g., hypogammaglobulinemia) |
| Ambiguous VDPV | aVDPV | Isolated from unimmunized persons or sewage without a clear transmission chain |
Pathogenesis / Why VDPV Emerges
The key driver is under-immunization in the community:
- OPV's live attenuated strains replicate in the gut and are shed in stool
- During prolonged replication (especially in individuals with inadequate immunity), mutations accumulate — particularly in the 5′ UTR and VP1 capsid protein — restoring neurovirulence determinants
- In immunocompromised hosts (especially B-cell deficiencies), the virus can replicate continuously for decades without being cleared
- Low community vaccine coverage allows sustained fecal-oral transmission and further evolution
Epidemiology
- Vaccine-derived polio was first significantly documented in Egypt, 1983–1993
- 385 cases occurred in Nigeria between 2005–2012
- 2020: 1,081 cases (peak, partly due to COVID-19 disruption of immunization)
- 2021: 682 cases
- 2023: 526 cases from 24 countries; 96% from Africa, 3% from the Eastern Mediterranean
- 92% of cases from 2018–2020 were type 2, a direct consequence of the 2016 switch from trivalent OPV (tOPV) to bivalent OPV (bOPV, types 1 & 3) — removing type 2 OPV immunity from new birth cohorts
The 2022 New York Case
A case of VDPV type 2 paralytic polio occurred in an unvaccinated adult in New York — the first US case of paralytic polio in nearly 40 years. The virus was genetically linked to wastewater collected ~1 month before and after. VDPV from OPV2 was also detected in wastewater from Canada, Israel, and the United Kingdom in 2022. — Harrison's, 22E
The tOPV → bOPV Switch (2016) and Its Consequences
Wild-type poliovirus type 2 was declared eradicated in 2015. In April 2016, the world simultaneously switched from trivalent OPV (types 1, 2, 3) to bivalent OPV (types 1 and 3). This:
- Built up a large cohort of children with no type 2 intestinal immunity
- Led to a surge in cVDPV2 outbreaks when residual OPV2 viruses continued to circulate
- Required emergency response with monovalent OPV type 2 (mOPV2), which itself risked generating new VDPV2 with suboptimal coverage
Novel OPV2 (nOPV2)
To break this cycle, a genetically stabilized novel oral polio vaccine type 2 (nOPV2) was developed and approved (via WHO Emergency Use Listing). It has:
- Modified 5′ UTR to reduce reversion to virulence
- A substantially lower risk of neurovirulence reversion
- Being deployed for outbreak response; the decline in VDPV type 2 cases in 2023 is attributed to its use — Harrison's, 22E; Aboshanab et al., 2026
Prevention & Control Strategies
| Strategy | Details |
|---|---|
| High OPV/IPV coverage | Primary prevention; herd immunity interrupts circulation |
| IPV-then-OPV sequential | At least 1 dose of trivalent IPV after bOPV reduces VDPV risk |
| Wastewater surveillance | Detects circulating VDPV before clinical cases appear |
| mOPV2 for outbreak response | Monovalent type 2 for targeted outbreak control |
| nOPV2 | Genetically stable; preferred over mOPV2 for new cVDPV2 outbreaks |
| Antivirals (investigational) | Pocapavir reduces shedding in iVDPV; combination therapy with monoclonal antibodies under development to manage long-term excretors |
Key Points for Eradication Concerns
Even if endemic wild-type polio is fully eradicated, VDPV poses a post-eradication threat because:
- Immunocompromised persons can shed VDPV for >25 years
- Wild-type poliovirus stocks in laboratories and manufacturing facilities remain
- Stopping OPV would eventually leave populations fully susceptible to any circulating VDPV
- IPV alone (used in industrialized countries) prevents paralysis but does not prevent gut replication/shedding — it does not confer intestinal (mucosal) immunity as effectively
— Harrison's Principles of Internal Medicine, 22E (2025)
Recent literature (Mohanty et al., 2023 — PMID 37283637; Aboshanab et al., 2026 — PMID 41389444) confirms the tOPV-bOPV switch as the primary driver of the cVDPV2 surge and highlights nOPV2 deployment as the current best response strategy.
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