Poliomyelitis, commonly known as polio, is a highly infectious disease caused by a virus that can invade the brain and spinal cord, potentially leading to irreversible paralysis. Global public health initiatives have successfully reduced the incidence of this devastating illness by over 99% since 1988, bringing the world to the brink of eradication. Despite this success, a rare and complex phenomenon exists where the vaccine used to combat the virus can, in specific circumstances, lead to a form of the disease. This risk is small but represents a significant challenge to the final goal of eliminating polio worldwide.
Understanding the Two Polio Vaccines
The world relies on two primary types of vaccines to prevent polio: the Inactivated Polio Vaccine (IPV) and the Oral Polio Vaccine (OPV). IPV contains a killed, or inactivated, version of the poliovirus and is administered by injection, typically into the arm or leg. Because the virus in IPV is non-living, it is incapable of replicating in the human body or causing the disease, meaning it carries no risk of vaccine-derived paralysis.
OPV, in contrast, is taken orally and contains a live, but weakened, or attenuated, form of the virus. This vaccine virus replicates briefly in the intestine, mimicking a natural infection and stimulating a strong immune response in the gut. This intestinal immunity blocks the virus from replicating and being shed, which helps stop transmission within communities. OPV is favored in many low-resource settings due to its low cost and ease of administration. The risk of vaccine-linked polio is exclusively associated with this live-attenuated OPV.
The Mechanism of Vaccine-Derived Paralysis
The mechanism by which OPV can lead to paralytic polio centers on the genetic instability of the attenuated virus strain. When a person receives OPV, the weakened virus replicates in the intestinal tract and is shed in the feces for several weeks. During this process of replication within the human gut, the live vaccine virus undergoes many cycles of multiplication.
In exceedingly rare instances, the weakened virus can genetically mutate, or revert, back toward a more neurovirulent form capable of causing paralysis. This mutated strain, known as a vaccine-derived poliovirus (VDPV), is then indistinguishable from the naturally occurring wild poliovirus. The genetic change involves the virus regaining the ability to travel to the central nervous system. This reversion event can affect the vaccinated individual or, more often, an unvaccinated person who comes into contact with the shed virus.
Comparing Vaccine-Associated Paralytic Polio and Outbreaks
The emergence of a neurovirulent strain from the OPV manifests in two distinct ways: Vaccine-Associated Paralytic Polio (VAPP) and Circulating Vaccine-Derived Poliovirus (cVDPV) outbreaks.
Vaccine-Associated Paralytic Polio (VAPP)
VAPP is an extremely rare event where the vaccinated individual or a close, susceptible contact develops paralysis directly linked to the recently administered OPV. This sporadic occurrence happens in approximately one case per 2.4 million to 2.7 million doses of OPV administered globally. VAPP is considered a one-time event that does not lead to sustained spread or an outbreak within the community.
Circulating Vaccine-Derived Poliovirus (cVDPV)
The more serious public health concern is cVDPV, which describes a reverted virus that has managed to sustain person-to-person transmission. An outbreak begins when the shed, mutated virus circulates in a population with low immunity for an extended period, allowing it to regain the capacity for widespread transmission and paralysis. These outbreaks are a clear signal of low immunization coverage because the virus can only circulate and evolve dangerously when it infects a large number of susceptible, unvaccinated people. The resulting cVDPV outbreaks resemble those caused by wild poliovirus and require an aggressive public health response to contain the spread.
Global Efforts to Eliminate Vaccine-Derived Polio
Acknowledging the rare risk associated with OPV, the global polio eradication strategy includes a phased approach to manage and eliminate vaccine-derived poliovirus. A major step was the coordinated global withdrawal of the Type 2 component of the trivalent OPV in 2016, as Type 2 was the strain most prone to reversion. This strategy, known as OPV cessation, aims to shift all routine immunization to IPV, which carries no risk of paralysis.
To manage the current threat of cVDPV outbreaks, particularly Type 2, a technological innovation known as novel Oral Polio Vaccine (nOPV) has been developed and deployed. This new vaccine is genetically stabilized to be significantly less likely to revert to a virulent form during replication. Since its rollout began in 2021, over a billion doses of nOPV2 have been administered in various countries under an emergency use listing. The ultimate success of the eradication effort depends on maintaining uniformly high vaccination coverage with IPV, ensuring that populations remain protected even as OPV is phased out entirely.