Nigeria is fighting an unusual outbreak of polio caused by a 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, which began last year, will be a new setback for eradication efforts in northern Nigeria. Vaccinations were halted there in 2003 for nearly a year because of rumors that they sterilized Muslim girls or contained the virus that causes AIDS. During that lull, polio spread to many countries, although most have eliminated the small outbreaks that resulted.
Officials have denied suggestions that they kept the outbreak a secret, and said that they had not realized until recently that as many as 70 of Nigeria's last 1,300 polio cases stemmed from a mutant vaccine virus rather than from a "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 WHO, said Wednesday.
The agency discussed the first 16 cases it knew of at meetings early this year and posted information on its Web site in April, "but only in places where lab people would look," he said.
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 an 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.
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