The new figure is double the 62 vaccine-derived cases reported in the country last year, and marks the continuation of the longest vaccine-derived polio outbreak ever seen, beginning in 2006, according to Oliver Rosenbauer, spokesman for World Health Organization's Polio Eradication Initiative.

The oral vaccine contains weakened live polio virus to build immunity in the patient, but can spread from a vaccinated person to the unvaccinated public and grow more virulent. If it is not contained and is allowed to mutate for months, it can grow strong enough to cause paralysis.

"Where you get vaccine-derived cases is in communities where the routine vaccination is very low," said Robert Scott, chair of the International PolioPlus Committee of the Rotary Foundation. Rotary International, the WHO, and the Centers for Disease Control and Prevention are part of a consortium of groups that support Nigeria's immunization drive.

"In this particular case in Nigeria, they stopped vaccinating for almost a year, so large areas were unvaccinated," Scott said.

Nigeria halted its polio vaccination program for more than a year in 2003 because of rumors the vaccine was a conspiracy against Muslims and that it caused infertility. With the encouragement of international health organizations, the country has worked to improve its coverage and implementation of polio vaccinations in recent years, but many remain unvaccinated.

Vaccine-derived cases of polio have been seen in 12 countries with a total of 383 cases reported worldwide over the last 10 years, according to Rotary figures. In that same period, some 10 billion doses of the vaccine were given to children, according to Scott.

The oral vaccine containing the live virus is used in Nigeria because it is the only vaccine that can eradicate the active disease, WHO's Rosenbauer explained.

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