Polio Disease and Poliovirus
There are three wild types of poliovirus (WPV) – type 1, type 2, and type 3. People need to be protected against all three types of the virus in order to prevent polio disease and the polio vaccination is the best protection. Type 2 wild poliovirus was declared eradicated in September 2015, with the last virus detected in India in 1999. Type 3 wild poliovirus was declared eradicated in October 2019. It was last detected in November 2012. Only type 1 wild poliovirus remains. There are two vaccines used to protect against polio disease, oral polio vaccine and inactivated poliovirus vaccine.
The oral poliovirus vaccine (OPV) is used in many countries to protect against polio disease and has been essential to the eradication effort. There are different types of oral poliovirus vaccine which may contain one, a combination of two, or all three different types of attenuated, or weakened, vaccine. Each has their own advantages and disadvantages over the others.
After wild poliovirus type 2 was declared eradicated in 2015, the world switched from trivalent to bivalent OPV (bOPV). Trivalent OPV contains all three types of poliovirus, while bOPV only contains poliovirus type 1 and 3. This switch means that OPV no longer protects against WPV2.
Though rare, when there is insufficient coverage in a community, the vaccine-virus may be able to circulate, mutate, and over the course of 12 to 18 months, cause paralysis.
Inactivated poliovirus vaccine (IPV) protects people against all three types of poliovirus. IPV does not contain live virus, so people who receive this vaccine do not shed the virus and cannot infect others, and the vaccine cannot cause disease. IPV does not stop transmission of the virus. OPV is used wherever a polio outbreak needs to be contained, even in countries which rely exclusively on IPV for their routine immunization program. Once polio has been eradicated, use of all OPV will need to be stopped to prevent re-establishment of transmission due to VDPVs. Countries that use bOPV have added a single dose of IPV to protect against WPV2.
Poliovirus containment is focused on eradicated polioviruses. WPV2 and WPV3 were declared eradicated in 2015 and 2019, respectively. Containment measures are being put in place for laboratories or other facilities that handle or store eradicated polioviruses.