Live vaccines, such as oral poliovirus vaccine; yellow fever vaccine; and measles, mumps and rubella vaccines, do not contain thiomersal, because it would kill the immunizing component.

In the meantime, more than 33 000 children were paralysed by wild poliovirus while more than 6.5 million polio cases were prevented by the oral polio vaccine.

A: a vaccine-derived poliovirus (VDPV) is a rare strain of poliovirus, genetically mutated from the strain contained in oral polio vaccine (OPV).