But say some children did not get the message that day, or their parents distrusted the vaccine and kept them home, or the vaccinators had to leave early. 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” mutant form keeps spreading because nearby villages are not fully vaccinated, it can, in a few cases — about one infection in 200 — paralyze a victim.
The name of that strain, circulating vaccine-derived poliovirus, gives the impression that it is contracted by being vaccinated. But that is not true.
“It’s not very well named, because it sounds like you get it from the vaccine,” said Heidi Larson, director of the Vaccine Confidence Project at the London School of Hygiene and Tropical Medicine. She added that there had been talk of altering the name to something less misleading, but that 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.”
