Poliomyelitis eradication

A public health effort to permanently eliminate all cases of poliomyelitis (polio) infection around the world began in 1988, led by the World Health Organization (WHO), the United Nations Children's Fund (UNICEF) and the Rotary Foundation. These organizations, along with the U.S. Centers for Disease Control and Prevention (CDC) and The Gates Foundation, have spearheaded the campaign through the Global Polio Eradication Initiative (GPEI), which helps to coordinate vaccination campaigns, environmental monitoring, evaluation of possible polio cases and logistics. Successful eradication of infectious diseases has been achieved twice before, with smallpox and bovine rinderpest. A public health effort to permanently eliminate all cases of poliomyelitis (polio) infection around the world began in 1988, led by the World Health Organization (WHO), the United Nations Children's Fund (UNICEF) and the Rotary Foundation. These organizations, along with the U.S. Centers for Disease Control and Prevention (CDC) and The Gates Foundation, have spearheaded the campaign through the Global Polio Eradication Initiative (GPEI), which helps to coordinate vaccination campaigns, environmental monitoring, evaluation of possible polio cases and logistics. Successful eradication of infectious diseases has been achieved twice before, with smallpox and bovine rinderpest. Prevention of disease spread is accomplished by vaccination. There are two kinds of polio vaccine—oral polio vaccine (OPV), which uses weakened poliovirus, and inactivated polio vaccine (IPV), which is injected. The 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 the OPV can also, in rare circumstances, induce polio or persistent asymptomatic infection in vaccinated individuals, particularly those that are immunodeficient. Being inactivated, the IPV is free of these risks but does not induce contact immunity. IPV is more costly and the logistics of delivery are more challenging. The 33 diagnosed wild polio virus (WPV) cases worldwide in 2018 represented a 95% reduction from the 719 diagnosed cases in 2000 and a 99.99% reduction from the estimated 350,000 cases when the eradication effort began in 1988. Of the three strains of polio virus, the last recorded wild case caused by type 2 (WPV2) was in 1999, and WPV2 was declared eradicated in 2015. Type 3 (WPV3) is last known to have caused polio on 11 November 2012, with all wild-virus cases since that date being due to type 1 (WPV1). All three types are represented among the periodic cases arising from mutated oral vaccine strains, so-called circulating vaccine-derived poliovirus (cVDPV). India is the latest country to have officially stopped endemic transmission of polio, with its last reported case in 2011. Three countries remain where the disease is endemic—Afghanistan, Pakistan and Nigeria. Eradication of polio has been defined in various ways—as elimination of the occurrence of poliomyelitis even in the absence of human intervention, as extinction of poliovirus, such that the infectious agent no longer exists in nature or in the laboratory, as control of an infection to the point at which transmission of the disease ceased within a specified area, and 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. 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: There are two distinct polio vaccines. The oral polio vaccine (OPV, or Sabin vaccine) contains an attenuated poliovirus, 10,000 times less able to enter the circulation and cause polio, delivered as oral drops or infused into sugar cubes. It is highly effective and inexpensive (about US$0.12 per dose in 2016) and its availability has bolstered efforts to eradicate polio. A study carried out in an isolated Eskimo village showed that antibodies produced from subclinical wild virus infection persisted for at least 40 years. Because the immune response to oral polio vaccine is very similar to natural polio infection, it is expected that oral polio vaccination provides similar lifelong immunity to the virus. Due to 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. It can also produce contact immunity. Attenuated poliovirus derived from the oral polio vaccine is excreted, and infects and indirectly induces immunity in unvaccinated individuals, 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 preferred vaccine of many countries, and it has long been preferred by the global eradication initiative. The primary disadvantage of the OPV derives from its inherent nature as an attenuated but active virus. It can induce vaccine-associated paralytic poliomyelitis (VAPP) in approximately 1 individual per every 2.4 million doses administered. Likewise, mutation during the course of persistent circulation in undervaccinated populations can lead to vaccine-derived poliovirus strains (cVDPV) that can induce polio at much higher rates. Until recently, a trivalent OPV containing all three virus strains was used, but with the eradication of wild poliovirus type 2 this was phased out in 2016 and replaced with bivalent vaccine containing just types 1 and 3, while use of monovalent type 2 OPV is restricted to regions with documented cVDPV2 circulation. The inactivated polio vaccine (IPV, or Salk) 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 (so-called fractional-dose IPV, fIPV) 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, a vaccinated individual is protected from contracting polio, but their intestinal mucosa can 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 for choice for industrialized, polio-free countries.