The ‘chicken and egg’ reason why polio outbreaks still happen

If polio is near extinction, why do outbreaks still pop up in places where the disease was thought to be long gone? The answer is complicated.

Global efforts to destroy disease-causing polioviruses have been quite a success story. Cases caused by the wild poliovirus have dropped 99% since 1988, thanks to vaccination efforts and a public-private partnership launched that year called the Global Polio Eradication Initiative.
Yet some immunization efforts carry the very rare risk of causing polioviruses to circulate in areas where many people might not yet be vaccinated or areas that were poorly vaccinated — an event that could lead to new cases of disease while trying to demolish it.
It turns out that live strains of poliovirus that are used in the oral poliovirus vaccine can mutate, spread and, in rare cases, even trigger an outbreak, representing a catch-22 in ongoing polio eradication endeavors.
These vaccine-derived viruses continue to haunt certain regions of the world.