A type of polio virus that had been wiped off the face of the earth has resurfaced in Nigeria a country already ravaged by the virus.
Type 2 poliovirus was eradicated in 1999, but a weakened form is used to make the oral polio vaccine (OPV). The outbreak in Nigeria which has so far caused over 100 cases of paralysis has been caused by vaccine virus that has mutated and regained its ability to infect people and cause disease.
Fears that the use of OPV could lead to a re-emergence of type 2 poliovirus led critics to call for a switch to the inactivated polio vaccine (IPV). But the benefits of OPV appeared to outweigh the risks; it is cheaper, easy to administer and proven to be effective in tropical countries.
Scientists also thought that mutated vaccine viruses were weaker than their wild counterparts something the Nigeria outbreak appears to be disproving. The first vaccine-derived type 2 polioviruses appeared in 2005 but started causing serious outbreaks only this year.
This could be because the virus mutated to become more transmissible or the number of children vulnerable to type 2 increased because vaccination campaigns focus on types 1 and 3, say experts.
It is hoped that an intensive OPV immunisation programme can stop the outbreak, but some argue that type 2 poliovirus will never be eradicated while it is being used in OPV.