On July 20, a 19-month-old boy was paralyzed in Guinea, West Africa, as a result of contracting poliovirus. He was then taken to Bamako, the capital of neighboring Mali, in search of treatment, where the outbreak of polio was confirmed. The virus he caught is a close match to that responsible for an outbreak in Guinea in August 2014, which indicates that this strain may have been circulating for a year.

The health authorities in both Mali and Guinea, alongside the WHO and the Global Polio Eradication Initiative will now act to immunize regions where vaccination levels are poor. 

While Africa has been free from wild poliovirus for a year, there are still victims (such as this Guinean boy) of the vaccine-derived poliovirus (VDPV). This version of the virus originates from the oral vaccine, which contains a live but weakened version of the poliovirus. The recipient of the oral vaccine gains immunity without suffering from the disease, but excretes the virus in their feces for a short time. This virus is incapable of infecting other people; in fact, it can pass through other humans and give them passive immunity. However in rare cases the virus can mutate and become infectious again. 

VDPV poses a serious risk if a community is under-vaccinated, but a fully vaccinated population will be protected from both wild poliovirus and vaccine-derived poliovirus. 

There is an injectable form of the polio vaccine, which contains inactivated virus rather than live virus. This therefore carries no risk of mutating into VDPV. However, it requires trained nurses and clean environments to perform the procedure, which aren't always available in developing nations. It is also five times more expensive than the oral vaccine, which can become a burden when vaccinating large portions of a country in a short space of time. 

The latest outbreak is possibly a result of reduced surveillance of the virus and decreased population immunity after the Ebola crisis in West Africa. Mali and Guinea have substantial vaccination gaps: national vaccination coverage in the former was 84% in 2014 but has dropped to 63% this year, while the latter is at even greater risk, with coverage dropping from 63% in 2013 and to 42% 2014. 

This news comes shortly after two outbreaks were confirmed in Ukraine, the first cases of polio in Europe for five years. Like Mali and Guinea, Ukraine suffers from low levels of vaccination which left its citizens vulnerable to the poliovirus. 

^{Featured Image: Julien Harneis/Flickr (CC BY-SA 2.0).}