If some children do not get the message that day, or their parents distrust the vaccine and keep them home, or the vaccinators have to leave early, they may still benefit. If any stool from vaccinated children contaminates local drinking water — or even a puddle that a child might splash in and then ingest — the virus can immunize other children, too.
Very rarely, however, the vaccine virus can mutate back into something resembling the wild kind. If that vaccine-derived mutation keeps spreading because nearby villages are not fully vaccinated, it can, in a few cases — about one infection in 200 — paralyze people.
The name of the polio strain may give the impression that people contract it from vaccinations, but that is not the case.
“It’s not very well-named,” said Heidi Larson, director of the Vaccine Confidence Project at the London School of Hygiene and Tropical Medicine. There has been talk of altering the name to something less misleading, she said, but any change would probably take too long.
While there has been success in Africa, there has been an increase in cases of wild poliovirus since 2018 in Afghanistan and Pakistan, where vaccinators are threatened with violence, and often killed.
“We need to look immediately at the most difficult circumstances, the most disadvantaged people, the most vulnerable people, the hardest to reach people — because that’s where we end up with the struggles at the end,” said Dr. Moeti, the W.H.O. director for Africa.
There is a parallel to be drawn with the coronavirus pandemic, she said.
“Those people who have the hardest life circumstances for one reason or another are the worst affected in terms of the mortality due to Covid-19,” she said, “and we are learning this lesson repeatedly.”