DAKAR, Senegal — Africa is set to announce that it has stamped out wild poliovirus after a three-decade campaign against a disease that once paralyzed 75,000 children on the continent every year.

The achievement is a major step toward ridding the globe of the virus that causes the disabling — and sometimes deadly — disease of polio: Only Afghanistan and Pakistan are still reporting cases.

“Future generations of African children can live free of wild poliovirus,” Dr. Matshidiso Moeti, the World Health Organization’s director for Africa, said in a briefing on Monday. He said 1.8 million cases of polio-related paralysis had been prevented over the past 24 years.

But the victory has a hollow note.

Every year, hundreds of people across Africa are still being infected with circulating vaccine-derived poliovirus, which can infect people in areas where there is only partial vaccination. African cases of the vaccine-derived strain, which results in the same symptoms as the wild kind, increased to 320 last year from 68 in 2018, and could rise again in 2020 because many vaccination campaigns were paused during coronavirus lockdowns.

The polio-free certification applies to what the World Health Organization calls the Africa region — one of its six global zones of operation — and it excludes North Africa. But no North African country has recorded a case of wild poliovirus since 2004, so the whole continent is now considered free of it.

The 46 presidents of the region (there would have been 47, but the president of Mali was recently deposed in a coup) will celebrate along with supporters of polio eradication efforts, among them the Microsoft founder Bill Gates and the Nigerian billionaire Aliko Dangote, on a video conference call on Tuesday.

The United States has been free of wild polio since 1979. The Global Polio Eradication Initiative started in 1988, with the aim of eradicating polio worldwide by the year 2000.

Nigeria was a major sticking point on the African continent. After northern Nigeria boycotted the vaccine in 2003 because of rumors about its safety, an outbreak there spread to 20 countries in five years.

A huge effort was initiated to change minds about the vaccine, and by 2015, it seemed that the situation was under control and rumors sufficiently squashed. But then, in 2016, four new cases of wild poliovirus were reported.

The state they came from, Borno, in northeastern Nigeria, was the site of a vicious insurgency by the extremist group Boko Haram, and getting to the people living there was extremely difficult, both because of the danger and because the Nigerian authorities frequently denied access for aid workers.

But after the four cases surfaced, the global health infrastructure swung into action. Nigeria’s president instructed the military to work with the vaccination teams. A mammoth effort began, using satellite imagery, data analysis and daring methods of getting to seemingly impossible-to-reach children.

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Credit…Sunday Alamba/Associated Press

Dr. Pascal Mkanda, the World Health Organization’s polio eradication coordinator for the Africa region, said: “There’s no program like the global eradication program. There’s no program which uses so much data, so much innovation.”

Vaccinators in Borno adopted a “hit and run” strategy.

When the military told them an area was safe, they would start a campaign there within 48 hours. More than 800,000 children were vaccinated this way in 2016. Others were vaccinated at markets on the edge of Boko Haram-occupied territory, when they came to buy provisions. When they went back to their communities, Dr. Mkanda said, the vaccine went through their digestive systems and out into the environment, and other children, who had not been inoculated, could then pick it up and also become immune.

Circulating vaccine-derived poliovirus spreads in the same way, except that the virus mutates over time and causes polio.

For example, say vaccinators go to a village to inoculate children.

The children line up by the church or market and get some drops squeezed into their mouths. The drops include a live but weakened version of the virus, which attaches to receptors in the intestines and is absorbed. The weakened virus cannot cause paralysis but teaches the child’s immune system to create antibodies that will fight the real thing if it were to come along.

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.”