If some children don't get the message that day, or if their parents distrust the vaccine and have to keep them at home or leave the vaccines early, they can still benefit. When stool from vaccinated children contaminates local drinking water – or even a puddle that a child may splash into and then ingest – the virus can immunize other children as well.
Very rarely, however, the vaccine virus can mutate back into something that resembles the wild species. If this vaccine-derived mutation continues to spread because surrounding villages aren't fully vaccinated, it can paralyze people in some cases – about one in 200 infections.
The name of the polio strain may give the impression that people can get it through vaccinations, but that's not the case.
"It's not very well known," said Heidi Larson, director of the Vaccine Confidence Project at the London School of Hygiene and Tropical Medicine. There was talk of changing the name to something less misleading, she said, but any change would likely take too long.
While there have been successes in Africa, cases of wild poliovirus have increased since 2018 in Afghanistan and Pakistan, where vaccines are threatened with violence and often killed.
"We need to look right now at the toughest circumstances, the most disadvantaged people, the most vulnerable people, the hardest-to-reach people – because that's where we end up fighting," said Dr. Moeti, the W.H.O. Director for Africa.
There is a parallel with the coronavirus pandemic, she said.
"Those people who, for one reason or another, have the most difficult living conditions are the worst at risk when it comes to mortality from Covid-19," she said, "and we are learning this lesson over and over."