Polio, or poliomyelitis, is a highly contagious disease caused by the poliovirus, primarily affecting children under five years old. It spreads through person-to-person contact, often via contaminated water or food, and can lead to severe complications including paralysis and, in rare cases, death. While significant progress has been made in eradicating wild poliovirus through global vaccination efforts, a rare form known as vaccine-derived polio can still occur. This form is linked to the oral polio vaccine and arises under specific circumstances.
What is Vaccine-Derived Polio
Vaccine-derived poliovirus (VDPV) is a strain of poliovirus that originates from the live, weakened virus in the oral polio vaccine (OPV). After vaccination, the weakened virus replicates in the intestines and is shed in feces, potentially spreading in communities with poor sanitation. In areas with low vaccination coverage, this weakened virus can circulate among under-immunized populations for extended periods, sometimes for 12 to 18 months. Over time, the virus can mutate and regain its ability to cause illness and paralysis, similar to wild poliovirus.
VDPVs are categorized into three main types based on their origin and circulation. Circulating vaccine-derived poliovirus (cVDPV) occurs with person-to-person transmission within a community, often leading to outbreaks. This is the most common form causing outbreaks and poses a public health risk comparable to wild poliovirus. Immunodeficiency-associated vaccine-derived poliovirus (iVDPV) is found in individuals with rare immune deficiencies who cannot clear the vaccine virus, leading to prolonged shedding and genetic changes. Ambiguous vaccine-derived poliovirus (aVDPV) refers to isolates from individuals without known immunodeficiency or from environmental samples where the source of circulation is unclear.
While VDPV can cause paralysis that is clinically indistinguishable from paralysis caused by wild poliovirus, its occurrence is extremely rare. Since 2000, over 10 billion doses of OPV have been administered globally, resulting in just over 1,000 reported cases of vaccine-derived polio paralysis. This rarity underscores the overwhelming benefits of polio vaccination in preventing widespread disease.
How Polio Vaccines Relate to Vaccine-Derived Polio
Two main types of polio vaccines exist: the Oral Polio Vaccine (OPV) and the Inactivated Polio Vaccine (IPV). The OPV contains live, weakened (attenuated) poliovirus strains, usually given as oral drops. This vaccine is highly effective at inducing immunity in the gut, which helps prevent the virus from replicating and spreading, offering both individual protection and community (herd) immunity. However, this live, weakened virus in OPV can, in very rare instances and primarily in under-immunized populations, mutate and regain its ability to cause paralysis, leading to VDPV.
IPV is an injectable vaccine made from inactivated (killed) poliovirus. It effectively prevents paralytic disease by triggering antibody production in the bloodstream, but it is less effective at preventing intestinal infection and viral shedding. A key advantage of IPV is that it carries no risk of causing vaccine-derived polio because it does not contain live virus. Historically, OPV was preferred for global eradication efforts due to its ease of administration, lower cost, and ability to provide widespread community immunity through shedding of the weakened virus.
The choice of vaccine type directly relates to the emergence of VDPV. In countries with high OPV vaccination rates, the weakened virus from the vaccine is quickly contained and eliminated. However, in areas with persistently low vaccination coverage, the prolonged circulation of the vaccine virus among unvaccinated individuals allows it to mutate and revert to a virulent form. This means that while OPV has been instrumental in dramatically reducing wild poliovirus cases, its use in inadequately immunized communities can inadvertently contribute to VDPV outbreaks.
Strategies to End Polio Globally
Global efforts to eradicate all forms of polio, including VDPV, are primarily led by organizations like the Global Polio Eradication Initiative (GPEI). These strategies involve a phased withdrawal of certain OPV types and an increased reliance on IPV. A significant step was the coordinated global switch in April 2016 from trivalent OPV (containing weakened forms of poliovirus types 1, 2, and 3) to bivalent OPV (containing only types 1 and 3). This change was made because wild poliovirus type 2 was eradicated in 1999, and the type 2 component of OPV was responsible for approximately 90% of all cVDPV cases.
The increasing use of IPV is another strategy, either as a standalone vaccine or in combination with OPV, to provide strong individual protection against paralysis without the risk of VDPV emergence. Many countries have introduced IPV into their routine immunization schedules. A promising development is the creation and deployment of novel oral polio vaccines (nOPV), specifically nOPV2 for type 2, which are genetically engineered to be more stable and less likely to revert to a virulent form.
Ultimately, achieving high vaccination coverage remains the most effective strategy to prevent both wild poliovirus and VDPV circulation. When a community reaches high immunity levels, the vaccine virus, even if it mutates, cannot find enough susceptible individuals to continue circulating and eventually dies out. Outbreaks of VDPV are typically managed with supplementary immunization activities, often involving targeted vaccination campaigns to boost immunity in affected areas.