Poliovirus is a highly contagious, non-enveloped RNA virus that belongs to the genus Enterovirus within the family Picornaviridae. There are three serotypes of poliovirus (PV1, PV2, and PV3), which primarily infect humans and can cause poliomyelitis or polio, a debilitating and potentially fatal disease.
Poliovirus is transmitted through the fecal-oral route or, less frequently, by ingestion of contaminated water or food. The virus initially infects the cells lining the throat and intestines, where it replicates. In most cases, poliovirus infection is asymptomatic or causes mild, flu-like symptoms. However, in approximately 0.5% of cases, the virus can spread to the central nervous system, particularly the spinal cord and brainstem, leading to paralysis or even death.
When poliovirus invades the nervous system, it preferentially targets motor neurons, leading to the death of these cells and resulting in muscle weakness or paralysis. The severity of the paralysis can vary, ranging from mild weakness to complete loss of muscle function. In some cases, paralysis of the respiratory muscles can cause respiratory failure, which can be fatal without medical intervention.
The development of effective vaccines has been instrumental in controlling and preventing polio. There are two types of polio vaccines:
- Inactivated poliovirus vaccine (IPV): Developed by Jonas Salk in the 1950s, IPV is based on inactivated (killed) poliovirus and is administered via intramuscular injection. IPV induces a strong antibody response, providing protection against all three serotypes of poliovirus without the risk of vaccine-associated paralytic poliomyelitis (VAPP).
- Oral poliovirus vaccine (OPV): Developed by Albert Sabin in the 1960s, OPV is a live, attenuated vaccine administered orally. OPV is highly effective in providing both individual and community immunity due to its ability to spread from vaccinated individuals to unvaccinated contacts. However, in rare cases, OPV can revert to a neurovirulent form, causing VAPP or circulating vaccine-derived polioviruses (cVDPV).
Due to widespread vaccination efforts led by the World Health Organization (WHO) and its partners, polio cases have decreased dramatically worldwide. In 1988, when the Global Polio Eradication Initiative (GPEI) was launched, polio was endemic in 125 countries, with more than 350,000 cases reported annually. As of 2021, wild poliovirus remains endemic in only two countries: Afghanistan and Pakistan. The goal of the GPEI is to achieve complete eradication of poliovirus, making polio the second human disease, after smallpox, to be eradicated.