Polio eradication


Polio eradication, the goal of permanent global cessation of circulation of the poliovirus and hence elimination of the poliomyelitis it causes, is the aim of a multinational public health effort begun in 1988, led by the World Health Organization, the United Nations Children's Fund and the Rotary Foundation. These organizations, along with the U.S. Centers for Disease Control and Prevention and The Gates Foundation, have spearheaded the campaign through the Global Polio Eradication Initiative. Successful eradication of infectious diseases has been achieved twice before, with smallpox in humans and rinderpest in ruminants.
Prevention of disease spread is accomplished by vaccination. There are two kinds of polio vaccine—oral polio vaccine, which uses weakened poliovirus, and inactivated polio vaccine, which is injected. OPV is less expensive and easier to administer, and can spread immunity beyond the person vaccinated, creating contact immunity. It has been the predominant vaccine used. However, under conditions of long-term vaccine virus circulation in under-vaccinated populations, mutations can reactivate the virus to produce a polio-inducing strain, while OPV can also, in rare circumstances, induce polio or persistent asymptomatic infection in vaccinated individuals, particularly those who are immunodeficient. IPV, being inactivated, does not carry these risks, but does not induce contact immunity. IPV is more costly and the logistics of its delivery are more challenging.
Nigeria is the latest country to have officially stopped endemic transmission of wild poliovirus, with its last reported case in 2016. Of the three strains of WPV, the last recorded wild case caused by type2 was in 1999, and WPV2 was declared eradicated in 2015. Type3 is last known to have caused polio in 2012, and was declared eradicated in 2019. All wild-virus cases since that date have been due to type1.
, Afghanistan and Pakistan are the only two countries where the disease is still classified as endemic. Recent polio cases arise from two sources, the original "wild" poliovirus, and the much more prevalent mutated oral vaccine strains, known as circulating vaccine-derived poliovirus or variant poliovirus. Vaccines against each of the three wild strains of polio have given rise to strains of cVDPV, with cVDPV2 being most prominent. cVDPV caused 312 confirmed paralytic polio cases worldwide in 2024, and was detected in 21 countries.

Factors influencing eradication of polio

Eradication of polio has been defined in various ways:
  1. As elimination of the occurrence of poliomyelitis even in the absence of human intervention.
  2. As extinction of poliovirus, such that the infectious agent no longer exists in nature or in the laboratory.
  3. As control of an infection to the point at which transmission of the disease ceased within a specified area.
  4. As reduction of the worldwide incidence of poliomyelitis to zero as a result of deliberate efforts, and requiring no further control measures.
In theory, if the right tools were available, it would be possible to eradicate all infectious diseases that reside only in a human host. In reality, there are distinct biological features of the organisms and technical factors of dealing with them that make their potential eradicability more or less likely. Three indicators, however, are considered of primary importance in determining the likelihood of successful eradication: that effective interventional tools are available to interrupt transmission of the agent, such as a vaccine; that diagnostic tools, with sufficient sensitivity and specificity, be available to detect infections that can lead to transmission of the disease; and that humans are required for the life-cycle of the agent, which has no other vertebrate reservoir and cannot amplify in the environment.

Strategy

The most important step in eradication of polio is interruption of endemic transmission of poliovirus. Stopping polio transmission has been pursued through a combination of routine immunization, supplementary immunization campaigns, and surveillance of possible outbreaks. Several key strategies have been outlined for stopping polio transmission:
  1. High infant immunization coverage with four doses of oral polio vaccine in the first year of life in developing and endemic countries, and routine immunization with OPV or IPV elsewhere.
  2. Organization of "national immunization days" to provide supplementary doses of oral polio vaccine to all children less than five years old.
  3. Active surveillance for poliovirus through reporting and laboratory testing of all cases of acute flaccid paralysis. Acute flaccid paralysis is a clinical manifestation of poliomyelitis characterized by weakness or paralysis and reduced muscle tone without other obvious cause among children less than fifteen years old. Other pathogenic agents can also cause AFP, such as enteroviruses, echoviruses, and adenoviruses.
  4. Expanded environmental surveillance to detect the presence of poliovirus in communities. Sewage samples are collected at regular and random sites and tested in laboratories for the presence of WPV or cVDPV. Since most polio infections are asymptomatic, transmission can occur in spite of the absence of polio-related AFP cases, and such monitoring helps to evaluate the degree to which virus continues to circulate in an area.
  5. Targeted "mop-up" campaigns once poliovirus transmission is limited to specific geographical foci.

    Vaccination

There are two distinct types of polio vaccine. Oral polio vaccine contains attenuated poliovirus. OPV is delivered as oral drops or infused into sugar cubes. It is highly effective and inexpensive and its availability has bolstered efforts to eradicate polio. The vaccine confers long-term, possibly lifelong, immunity to the virus. Because of its route of administration, it induces an immunization of the intestinal mucosa that protects against subsequent infection, though multiple doses are necessary to achieve effective prophylaxis. Attenuated poliovirus derived from the oral polio vaccine is excreted, infecting and indirectly inducing immunity in unvaccinated individuals, and thus amplifying the effects of the doses delivered. The oral administration does not require special medical equipment or training. Taken together, these advantages have made it the favored vaccine of many countries, and it has long been preferred by the global eradication initiative.
The primary disadvantage of OPV derives from its inherent nature. As an attenuated but active virus, it can induce vaccine-associated paralytic poliomyelitis in approximately one individual per every 2.7million doses administered. The live virus can circulate in under-vaccinated populations and over time can revert to a neurovirulent form causing paralytic polio. Until recent times, a trivalent OPV containing all three virus strains was used, but with the eradication of wild poliovirus type2 this was phased out in 2016 and replaced with bivalent vaccine containing just types 1 and 3, supplemented with monovalent type2 OPV in regions with documented cVDPV2 circulation. A novel OPV2 vaccine genetically modified to reduce the likelihood of disease-causing activating mutations was granted emergency licencing in 2021, and subsequently full licensure in December 2023. Genetically stabilsed vaccines targeting poliovirus types 1 and 3 are in development, with the intention that these will eventually completely replace the Sabin vaccines.
The inactivated polio vaccine contains trivalent fully inactivated virus, administered by injection. This vaccine cannot induce VAPP nor do cVDPV strains arise from it, but it likewise cannot induce contact immunity and thus must be administered to every individual. Added to this are greater logistical challenges. Though a single dose is sufficient for protection, administration requires medically trained vaccinators armed with single-use needles and syringes. Taken together, these factors result in substantially higher delivery costs. Original protocols involved intramuscular injection in the arm or leg, but recently subcutaneous injection using a lower dose has been found to be effective, lowering costs and also allowing for more convenient and cost-effective delivery systems. The use of IPV results in serum immunity, but no intestinal immunity arises. As a consequence, vaccinated individuals are protected from contracting polio, but their intestinal mucosa may still be infected and serve as a reservoir for the excretion of live virus. For this reason, IPV is ineffective at halting ongoing outbreaks of WPV or cVDPV, but it has become the vaccine of choice for industrialized, polio-free countries.
While IPV does not itself induce mucosal immunity, it has been shown to boost the mucosal immunity from OPV, and the WHO now favors a combined protocol. It is recommended that vulnerable children receive a dose of OPV at birth, then beginning at the age of six weeks a 'primary series' consisting of three OPV doses at least four weeks apart, along with one dose of IPV after 14 weeks. This combined IPV/OPV approach has also been used in outbreak suppression.

Herd immunity

Polio vaccination is also important in the development of herd immunity. For polio to occur in a population, there must be an infecting organism, a susceptible human population, and a cycle of transmission. There is no animal reservoir for poliovirus, and the transmission cycle of polio is from one infected person to another person susceptible to the disease, usually through the fecal-oral route. If the vast majority of the population is immune to a particular agent, the ability of that pathogen to infect another host is reduced; the cycle of transmission is interrupted, and the pathogen cannot reproduce and dies out. This concept, called community immunity or herd immunity, is important to disease eradication, because it means that it is not necessary to inoculate 100% of the population—a goal that is often logistically very difficult—to achieve the desired result. If the number of susceptible individuals can be reduced to a sufficiently small number through vaccination, then the pathogen will eventually die off.
Herd immunity is an important supplement to vaccination. According to the concept of herd immunity, the population for whom the vaccine fails is still protected by the immunity of those around them, and it can only be achieved when vaccination levels are high. It is estimated that the minimum herd immunity threshold for poliovirus eradication ranges from approximately 75% in wealthy high-hygiene populations to 97% in poorer environments. If routine immunization were to be stopped, the number of unvaccinated, susceptible individuals would soon exceed the capability of herd immunity to protect them.