A report in an American medical journal that children in northern Nigeria have been infected with polio by the vaccine designed to prevent it have raised fears that Nigeria’s already lagging polio prevention efforts could be further delayed.

Such vaccine-derived outbreaks have occurred previously in other parts of the world, usually in regions where there is low polio immunisation coverage, but the 69 cases recorded in Nigeria are the largest on record, the scientists said in the study.

The finding could be a “serious setback” for the global polio eradication campaign, because it is occurring in a region where rumours about vaccine safety “derailed vaccination efforts” several years ago, scientists warned in the study, released on 28 September by the US Centre for Disease Control (CDC) in its publication Morbidity and Mortality Weekly.

Nigeria accounts for at least 70 percent of all new polio cases worldwide, while almost all other countries in the world have successfully eradicated the disease.

The CDC’s results have been reported in the leading scientific journal Science Magazine and confirmed by the World Health Organization (WHO). But a spokeswoman for the WHO’s Polio Eradication Imitative pointed out that around 2,000 cases of polio were reported in the same area at the same time and they were not caused by the vaccine.

“All vaccines have risks associated with them. From a public health perspective it is important for people to weigh the risks with the benefits,” spokeswoman Sona Bari told IRIN on 3 October. “We hope that people don’t focus now on the vaccine as the problem when it is the disease that still poses a far greater problem.”

Polio mainly affects children under the age of five. Once contracted, it can disable people for life. Polio cannot be cured though vaccination can provide a lifetime protection.

The vaccine, like the disease, is passed on through faecal matter. Bari said that many of the children who contracted the vaccine-derived polio may have never been administered the vaccine but rather ingested it indirectly.

“The vaccine mutated and they had no other vaccine to protect them,” she said.

That may be difficult for many people in northern Nigeria to understand. Radical Muslim radical religious leaders there have made unfounded claims that the polio vaccine was laced with agents causing AIDS and sterility. As a result three northern Nigeria states suspended polio vaccination in 2004, including Kano State which halted vaccinations for nearly a year, resuming only after tests confirmed the vaccines were safe.

Nigerian and international officials said people in northern Nigeria had recently started to accept vaccination though the uptake is still low in some areas. Currently six northern states - Kano, Kaduna, Sokoto, Zamfara, Kebbi and Jigawa - account for over 70 percent of all polio cases in the country.

Nigerian health officials say they are concerned that the progress that has been made will be undermined when people learn that the polio vaccine can cause the disease. "These cases seem to confirm fears that that there may be something unwholesome about the polio vaccines and could lead to further boycott of vaccination," a senior official of the National Programme of Immunisation in Nigeria's Health Ministry told IRIN on condition of anonymity.

"Yet the fact remains that these infections were the consequence of low immunisation coverage caused by the initial boycott in the north,” he said. “So it poses a major public health advocacy challenge to make the people understand the real facts and that it calls for more not less vaccination," he added.