• Wild poliovirus (WPV) comprises 3 serotypes (1, 2, 3) which may infect and destroy spinal cord lower motor neurons. PV is shed via salivary droplets and feces and it is transmitted person-to person.
  • A case of polio (paralysis) occurs “only” among ~200 (WPV type 1) to ~1000 (type 2 or 3) infections. Thus, one case is the tiny ‘tip of the iceberg' of widespread infection.
  • Infection induces long-lasting type-specific immunity, protecting from risk of disease when re-infected, but not from re-infection per se.
  • In low-income countries, polio occurs early in life and immunity plateaus at age 5 years with almost 100%; in richer countries the age of polio is shifted towards older ages since the 1940s.
  • While the majority of WPV-infected persons remain asymptomatic a small proportion has short fever with upper respiratory or gastrointestinal symptoms.
  • In a few subjects, this first phase may be followed by an acute onset of paralysis of skeletal muscles, due to loss of lower motor neurons from PV infection (=poliomyelitis) with a case-fatality rate of 5% - 20%; bulbar involvement increases risk of death, cortical functions (other than emotional) are unaffected.
  • Recurrence of pain and worsening of residual motor power may occur 3-4 decades later ("post-polio syndrome").
  • With no specific treatment available, prevention with one of 2 basic vaccine types (live = oral polio vaccines (OPV); and inactivated (IPV) whole cell vaccine) is of highest importance.
  • With IPV, 3 primary doses and one booster protect nearly 100%, whereas 2 priming and 1 booster doses are sufficient to protect >95% of subjects, provided the first dose is given at or after 8 weeks of age.
  • OPV is given orally to induce systemic and gut mucosal immunity, following intestinal infection by vaccination.
  • In the USA and in most temperate regions one dose induces protective immunity in ~75% subjects against the 3 virus types and the immunity gap is closed by 2 additional doses.
  • In tropical/developing countries vaccine efficacy is as low as ~10% for types 1 or 3 and it may take 10-15 doses to induce immunity in >90%.
  • While there is no safety concern with IPV, with OPV attenuating mutations may revert, rarely resulting in ‘vaccine-associated paralytic poliomyelitis' (VAPP), clinically indistinguishable from WPV-caused polio.
  • VPV can spread and cause VAPP in susceptible contacts. In 2010 only 4 countries continue to have indigenous transmission of WPV 1 or 3
  • Transmission of type 2 had been globally interrupted in 1999.
  • Nearly all rich countries have abandoned OPV in favor of IPV in order to avoid VAPP.