Officials of the World Health Organization fear that news of the outbreak will be a new setback for eradication efforts in northern Nigeria, where vaccinations were halted in 2003 for nearly a year because of rumors that the vaccine sterilized Muslim girls or contained the AIDS virus. During that lull, polio spread to many new countries, although most have snuffed out the small outbreaks that resulted.

Officials deny suggestions that they kept the outbreak, which began last year, a secret, and say that they did not realize until recently that as many as 70 of Nigeria’s last 1,300 polio cases stemmed from a mutant vaccine virus rather than “wild type” virus, which causes most polio.

“It was an oversight on our part,” Dr. Bruce Aylward, director of the polio eradication campaign for the W.H.O., said yesterday. The agency discussed the first 16 cases it knew of at meetings early this year and posted information on its Web site in April, he said, “but only in places where lab people would look.”

Outbreaks of vaccine-derived polio are unusual but not unheard of. Individual cases have been known for years. For example, a former lieutenant governor of Virginia was partly paralyzed in 1973, apparently after changing the diapers of his son, who had received oral vaccine.

The first spreading outbreak of a vaccine-derived strain, in which 22 children were paralyzed, was detected in 2001 in the Dominican Republic and Haiti.

Experts now believe another took place in Egypt in the late 1980s but went unnoticed amid the much larger numbers of wild-type infections. There have been others in the Philippines, Madagascar, China and Indonesia.

All were eventually eliminated by immunizing more children, and experts argue that the latest outbreak was able to spread because, until recently, only 30 to 40 percent of the children in northern Nigeria were vaccinated. About 70 percent are vaccinated there now, Dr. Aylward said.

In 2000, the United States switched to injected vaccine made from killed virus, which cannot mutate. But oral drops with the live, weakened version of the virus are still used in most poor countries, including those where the disease has never been eliminated: Nigeria, India, Pakistan and Afghanistan.

This vaccine, invented by Albert Sabin, is easier to give, offers much stronger protection and can beneficially “infect” other family members or neighbors, protecting them too.

But in rare cases, it can mutate into something resembling wild polio virus, which can paralyze or kill. Dr. Aylward pointed out that 10 billion doses of oral vaccine had been given in the last 10 years, so such mutations are presumably extremely unusual.

Polio often circulates undetected; in only one of 200 infections will it cause paralysis, which signals health officials to look for the virus in the area.