What is vaccine-derived polio?

• Oral polio vaccine (OPV) contains a weakened version of poliovirus, activating an immune response in the body. A vaccinated person transmits the weakened virus to others, who also develop antibodies to polio, ultimately stopping transmission of poliovirus in a community.
• In very rare instances, the virus in the vaccine can mutate into a form that can paralyse - this is what is known as a vaccine-derived poliovirus or VDPV.
  
• If this virus regains the ability to circulate, it is called a circulating vaccine-derived poliovirus (cVDPV). As with naturally occurring poliovirus, the only protection against cVDPV is full vaccination.
• The spread of a cVDPV shows that too many children remain under-immunized. A fully-immunized population will be protected from all strains of poliovirus, whether wild or vaccine-derived.
• Over the past 10 years, more than 10 billion doses of OPV have been administered to more than 2.5 billion children. As a result more than 3.5 million polio cases were prevented. During that time, 18 outbreaks of cVDPVs have occurred in 16 countries, resulting in 510 VDPV cases.