http://www.sciencemag.org/news/2019/12/polio-eradication-program-faces-hard-choices-endgame-strategy-fails

Polio vaccinators in northwestern Nigeria, where the live-virus vaccine itself is causing new infections.

The “endgame” in the decadeslong campaign to eradicate polio suffered major setbacks in 2019. While the effort lost ground in Afghanistan and Pakistan, which recorded 116 cases of wild polio—four times the number in 2018—an especially alarming situation developed in Africa. In 12 countries, 196 children were paralyzed not by the wild virus, but by a strain derived from a live vaccine that has regained its virulence and ability to spread. Fighting these flare-ups will mean difficult decisions in the coming year.