Jul 3, 2013

Oral polio vaccine (OPV) contains an attenuated (weakened) vaccine-virus, activating an immune response in the body.

If a population is seriously under-immunized, an excreted vaccine-virus can continue to circulate for an extended period of time. The longer it is allowed to survive, the more genetic changes it undergoes. The vaccine-virus can genetically change into a form that can paralyse – this is what is known as a circulating vaccine-derived poliovirus (cVDPV).