Cases of polio have been found in Mali and Guinea, bringing to 12 the number of African countries recently reinfected by an ongoing polio epidemic in Nigeria.

The new cases suggest the effect of northern Nigeria's 16-month boycott of a global polio-eradication program is not over even though that region resumed immunization efforts last month. A World Health Organization official said yesterday that it is likely polio is also being transmitted undetected in Liberia and Sierra Leone.

"Our modus operandi has got to be that we assume these places could be infected, too," said R. Bruce Aylward, the global coordinator of the eradication initiative, which is in its 16th year.

At the end of 2002, polio existed in three Asian and three African countries. Over the past year, however, 10 formerly polio-free countries in Africa recorded cases traceable by genetic fingerprinting to the populous state of Kano in Nigeria's Muslim north. Worldwide, 602 cases of polio have been found this year, 476 in Nigeria.

One in every 200 polio infections causes paralysis, but in most cases, the infection causes little or no noticeable illness. The virus multiplies in the intestine, is excreted for weeks in feces and spreads most easily in poor, crowded populations without sanitation. In addition to two cases in Mali and one in Guinea, WHO officials reported that three more cases have been found in the Darfur region of Sudan, where thousands of people live in refugee camps. A case there in June marked polio's first appearance in Sudan in three years. Mali and Guinea recorded their last polio cases in 1999.

Polio immunization ceased in Kano and several neighboring states after rumors circulated that the vaccine contained anti-fertility agents and that polio eradication was part of an anti-Muslim agenda. Rejection of polio vaccination by many local leaders was also part of an internal political struggle between the Muslim north and the Christian south of Nigeria.

Many leaders, including Kano's governor, recently endorsed resumption of vaccination after elaborate efforts to demonstrate the safety of the vaccine. They included tests of its purity, a visit by opponents to vaccine manufacturing sites overseas and a commitment by UNICEF and WHO to provide vaccine made in Indonesia, a Muslim country, to Nigerian states that requested it.

Kano, which has 3 million children in the under-5 age group that is the target of polio vaccination, resumed population-wide immunization at the end of July. Aylward said 50 to 75 percent of children were reached, depending on the village and neighborhood. In general, at least 80 percent of children must be immune for the person-to-person chain of transmission to be broken.

Vaccinators reported "much less resistance [to immunization] than they were led to believe," Aylward said. "People valued the opportunity to vaccinate their children because they were so aware of the burden of disease."

Nigeria, which had hoped to stop polio transmission by the end of 2004, is planning immunization activities with a target of halting the virus by the end of next year. Two coordinated immunization campaigns will be held in 22 African countries this October and November, before and after the Muslim holy month of Ramadan. In all, 74 million children will be targeted.

To date, about $3.1 billion has been spent on polio eradication.

In Kano, Nigeria, widespread vaccinations resumed in July.