Oral polio vaccine (OPV) contains an attenuated (weakened) vaccine-virus…
When a child is immunized with OPV, the weakened vaccine-virus replicates in the intestine for a limited period, thereby developing immunity by building up antibodies. During this time, the vaccine-virus is also excreted. In areas of inadequate sanitation, this excreted vaccine-virus can spread in the immediate community (and this can offer protection to other children through ‘passive’ immunization)…
[T]he vaccine-virus can genetically change into a form that can paralyze – this is what is known as a circulating vaccine-derived poliovirus (cVDPV).
Since 2000… 24 cVDPV outbreaks occurred in 21 countries…