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.