Some 13 million children in northern Nigeria are being vaccinated against polio in a bid to wipe out the disease.
Nigeria is one of six countries where polio is endemic
About 250,000 technicians are making house calls to reach all children under five in eight states, which have become the world's polio epicentre.
Last year, an immunisation campaign was halted in northern Nigeria after Islamic clerics said it was unsafe.
Nigeria now accounts for about 80% of new cases and 12 previously polio-free African nations have been reinfected.
Vaccinations were restarted in July after the clerics were satisfied with vaccines shipped in from Indonesia.
But July's campaign only reached some 60% of the region's children.
The clerics, based in northern Nigeria's biggest city Kano, had said that the original vaccine was part of a US plot to make Muslim women infertile.
This was strongly denied by the World Health Organization (WHO).
Once the new programme is completed, Nigeria will prepare for a synchronised vaccination campaign - which will run in 22 countries - at the beginning of October as part of final efforts to rescue the WHO goal of wiping out polio worldwide by 2005.
Polio, which mainly affects children under five, is caused by a virus that invades the nervous system and can cause paralysis or death.
The disease is considered endemic in only six countries - Afghanistan, Egypt, India, Niger, Nigeria, and Pakistan.
When the eradication drive was launched in 1988, polio was a serious problem in 125 countries.