Today, on World Polio Day, wild poliovirus type 3 has been declared eradicated by a commission of the World Health Organization. The last case of type 3 poliomyelitis was recorded in 2012 in Nigeria. Because wild poliovirus type 2 was declared eradicated in 2015, now only wild poliovirus type 1 continues to circulate, causing paralysis in Afghanistan and Pakistan.
Wondering why the eradication certificate (pictured) says ‘wild’ poliovirus, and not simply poliovirus? The reason is that bivalent oral poliovirus vaccine, containing types 1 and 3, continues to be used globally. After oral administration of this vaccine, vaccine-derived strains are excreted in the feces. Although wild poliovirus type 2 was declared eradicated in 2015, vaccine-derived type 2 poliovirus strains continue to circulate. These vaccine-derived viruses have so far in 2019 caused 102 cases of poliomyelitis.
In other words, we will not be able to declare that poliovirus is eradicated until we stop using the oral poliovirus vaccines. Use of the type 2 oral poliovirus vaccine was stopped in 2016, and WHO suggested that at least one dose of inactivated poliovirus vaccine (IPV) – containing all three serotypes – be included in immunization schedules. Lower IPV use than anticipated has led to continued circulation of vaccine-derived type 2 poliovirus. Compounding the problem is the use of OPV to control outbreaks of vaccine-derived paralysis, leading to introduction of more vaccine viruses into the environment.
Vaccine-derived type 1 poliomyelitis is quite rare, and so if we can vaccinate properly in Afghanistan and Pakistan, we will likely be able to eradicate this serotype. The solution to the problem of type 2 poliovirus will require a complete global switch to IPV, or the use of a new vaccine that cannot revert during replication in the gut and cause paralysis. Such vaccines are in development.