Nigeria is fighting an unusual outbreak of polio caused by mutating polio vaccine, world health authorities say, but the only remedy is to keep vaccinating children there.

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.”