Teams were to spread out across 23 countries in sub-Saharan Africa on Friday to begin what global health officials hope is the final push against the crippling polio virus, with the goal of vaccinating 80 million children, including those in northern Nigeria, where opposition had derailed the eradication effort.

One million health workers and volunteers are to travel over five days by riverboat, motorbike, horseback and four-wheel-drive truck as they seek to deliver the vaccine to the most remote jungle villages and the densest urban slums. Officials say it will be the largest coordinated public health effort in Africa's history.

The urgency of reaching nearly all children under 5 years old in every vulnerable region became clear over the past two years as opposition in northern Nigeria, where vaccination efforts were temporarily halted, allowed the virus to spread there and to 12 countries where it had been eliminated.

In northern Nigeria, which is predominantly Muslim, rumors circulated that the vaccine caused sterility and spread HIV, the AIDS virus. But the concerns have been eased by purity tests and fact-finding trips to factories in Muslim countries that make the vaccine, health officials said.

In a series of public events, Nigerian political and religious leaders personally placed drops of the vaccine on the tongues of their children. Last week, President Olusegun Obasanjo administered it to the year-old daughter of the governor of Kano, the northern state where opposition to the vaccine has been strongest.

"To ensure a vital future for our country, we have to strive for a polio-free Nigeria through effective mass mobilization and community participation," Obasanjo said, according to news reports.

Problems in Nigeria have forced health officials to push back their deadline for eradicating polio, once set for 2000, to December 2005. But they said mass immunizations this weekend and a series of follow-ups in November and next year could complete an effort that already has involved decades of work and billions of dollars.

"It is absolutely do-able," said Mohammed Belhocine, the World Health Organization's top official in Nigeria. "I feel much more hopeful now than even two months ago."

WHO is leading the eradication effort, along with UNICEF, Rotary International and the U.S. Centers for Disease Control and Prevention.

One of the foremost critics of the vaccination effort, Datti Ahmed, a physician in Kano, has said little publicly for months. Reached by telephone, he acknowledged that he had withdrawn from the debate.

"I'm not going to talk to you about polio. . . . It's up to the people to decide what they want to do," he said before abruptly hanging up.

Polio enters the body through the mouth and reproduces in the intestines of its hosts. It cripples about one of every 200 people who contract the disease but is easily and rapidly transmitted by those infected, particularly in places with poor sanitation.

The virus was long a global scourge but has been eliminated from developed countries since Jonas Salk created the first polio vaccine in the early 1950s. Most developing countries still struggled with the disease until recent years. In 1988, when the global eradication effort formally began, there were 350,000 cases in 125 countries worldwide.

Last year, the total was down to 784 reported cases, and polio was endemic in only Nigeria, Niger, Egypt, Pakistan, Afghanistan and India. All six have active eradication campaigns, with the goal of eliminating polio by the end of next year.

When the Nigerian strain revived last year, it hopscotched thousands of miles across the continent, appearing this year in such far-flung places as Botswana in southern Africa, Ivory Coast in West Africa and Sudan in eastern Africa.

The epicenter remains Nigeria, which has experienced three-quarters of the world's 786 polio cases so far this year, according to WHO figures. Neighboring Niger, the only other sub-Saharan country where the virus is regarded as endemic, has had 20 cases.

After Nigerian officials made their fact-finding trip to vaccination factories, the effort resumed in Kano. The initial round in July and August was partially successful, with a vaccination rate of 58 percent. Scientists say 80 percent of a population must be vaccinated repeatedly to halt transmission.

Health workers reported being taunted by children and chased away from some homes, according to news accounts. Other parents refused the vaccine, or hid their children.

"A lot of damage was done," said Samaila Muhammad Mera, a traditional ruler in northern Nigeria and a spokesman for the sultan of Sakoto, a senior Muslim leader who has backed the vaccination effort. "It will take time to build confidence again. But I believe it is beginning."

Health officials have 100 million doses of the vaccine, which consists of weakened but still living virus, on hand for the push. The vaccination teams will carry the doses in ice-cooled containers across countries where daytime temperatures often exceed 100 degrees.

If the effort is successful, polio would be the second disease ever eradicated. The first was smallpox.

Nigerian President Olusegun Obasanjo has backed the vaccination effort.