A case of vaccine-derived polio has paralyzed a child in Bamako, the capital of Mali, and an emergency vaccination drive is being organized to forestall an outbreak, the World Health Organization announced Monday.

It is the first time the disease has been seen in Mali since 2011. The patient is a Guinean child whose parents traveled to Bamako seeking medical care. The child’s virus is a close genetic match to a strain last detected in a nearby region of Guinea in 2014.

The child was not infected with the “wild type” polio virus, which the W.H.O. says has been eliminated from Africa. No cases of paralysis from wild virus have been found in Africa in a year, but it takes three years before a region is declared polio-free.

Rather, it was a strain created when one of the three live, weakened virus strains in the oral polio vaccine mutated to become dangerous again. Such mutations are a rare but persistent consequence of relying on oral polio vaccine drops, which are easy to administer and much more protective than so-called killed vaccine.