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Polio

Poliomyelitis (polio) is a highly infectious viral disease transmitted by contaminated drinking water and sewage. There are three types of wild polioviruses: serotype 1, 2, and 3. The polioviruses replicate in the gastrointestinal tract and can invade the nervous system, which can cause permanent limb paralysis. While polio can strike at any age, it mainly affects children under age 5. Vaccination against polio is the best way to prevent polio disease. There are two types of vaccines currently used against polio—a live attenuated oral polio vaccine (OPV) delivered as oral drops, and inactivated polio vaccine (IPV) delivered via injection. Due to global polio vaccinations and improved sanitary conditions, the world is very close to eradicating polio. Over 2.5 billion children have been vaccinated since 1988 and the number of polio cases per year is down by 99 percent. Currently the poliovirus serotype 2 has been eradicated globally, type 3 virus has not been detected globally since late 2012, and type 1 causes a few hundred cases worldwide. While there has been remarkable progress in polio eradication, more affordable vaccines are needed to complete the job of eradicating wild type polio and maintain protection post polio eradication.

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On the PATH website

Polio disease

  • Polio is a highly infectious disease caused by poliovirus that invades the nervous system and can cause permanent paralysis.
  • There are three serotypes of poliovirus (1, 2, and 3). Immunity to one serotype does not protect against the other serotypes.
  • Wild poliovirus type 2 was eradicated more than a decade ago, type 3 has not been detected since late 2012, and there are a few hundred cases of type 1 worldwide.
  • Polio can strike at any age but it mainly affects children under age 5.
  • The world is very close to eradicating polio. Over 2.5 billion children have been vaccinated since 1988 and the number of polio cases per year is down by 99 percent.

Polio vaccines

  • There are two types of vaccines against polio—a live attenuated oral polio vaccine (OPV) delivered as oral drops and inactivated polio vaccine (IPV) delivered via injection.
  • There are two forms of OPV vaccines—a trivalent vaccine that contains attenuated (weakened) viruses for serotypes 1, 2, and 3, and a bivalent OPV for serotypes 1 and 3, which is planned to be introduced in April 2016. The live attenuated polio vaccine viruses can also shed from immunized individuals and circulate as circulating vaccine-derived poliovirus (cVDPV) in a community.
  • Among the cVDPV, the serotype 2 cVDPV are the most common with over 90% circulating strains of this serotype. The cVDPV will continue unless global community makes the transition from OPV to IPV after wild poliovirus is eradicated.
  • The live attenuated vaccine virus can mutate in some cases, and result in vaccine-associated paralytic polio (VAPP), although this is rare, occurring once for every 2.7 million first doses of the vaccine.
  • In developed countries where poliovirus is eradicated, only IPV vaccine is used in childhood immunization. The current price of IPV (>$3/dose) is out of reach for low- and middle-income countries, and they rely mostly on OPV ($.14 -.17/dose) for mass vaccination campaigns.
  • In order to accomplish complete global polio eradication and to ensure protection against poliovirus post eradication, new vaccines and surveillance tools are needed:
    • Lower cost OPV for maintaining population immunity, for stamping out pockets of endemic wild poliovirus transmission, and for controlling outbreaks.
    • Much more affordable IPV to contain and end VDPVs and sustain population immunity against poliovirus post eradication.
    • Affordable IPV-containing pediatric combination vaccines for future birth cohorts in low-income countries.

References

Page last updated: April 2015.

Photo: PATH/Gabe Bienczycki.