The charity Polio Eradication explain further: “The vaccine virus can be spread from child to child. This helps spread the protection to other children, even those who are unvaccinated.
“However, when a large number of children are not vaccinated, and immunity levels are very low the vaccine virus can continue to spread from one unprotected child to another for a long period of time. As the virus continues to find unvaccinated children it undergoes small genetic changes.
On very rare occasions, this can cause the weakened virus to change into a form which can cause paralysis. These rare virus strains are called vaccine-derived polioviruses. If immunity is high, vaccine derived polio cannot arise.”
Vaccine derived polio arises when the vaccination rate and immunity levels are low; this is a situation facing parts of the UK with low polio vaccination rates.
Source: | This article first appeared on Express.co.uk