Fifty three million children, including every child in Nigeria, are being targeted by a mass immunization drive against polio in eight west African countries, the UN Children's Fund said.
More than 162,000 people were being mobilised to administer the oral vaccines to under five year-olds in two rounds starting from Friday in Benin, Burkina Faso, Ivory Coast, Mali, Niger, Nigeria, and Togo.
A first round of vaccinations in the 67 million dollar campaign against the crippling polio virus was carried out in Ghana earlier this month, UNICEF said.
"The highest priority is to reach every child in Nigeria, which is one of the four endemic countries, and in the high risk areas across the region," UNICEF spokeswoman Miranda Eeles told journalists.
Half of the 66 million doses of polio vaccine used in each round are destined for Nigeria, a hotbed for the virus.
The campaign is aimed at stopping the spread of polio following an outbreak in Nigeria last year that reached into to six African countries, and the reinfection of Niger in 2007.
Some 844 polio cases were reported in the eight countries in 2008, 95 percent of them in Nigeria, Africa's most populous country, according to UNICEF.
Nigeria's Kano state has been the epicentre of transmission of the virus to other parts of the world in recent years.
In 2003, state authorities there stopped polio immunization for a year after radical Muslim clerics and some doctors said the vaccine was designed to render girls infertile as part of a US-led plot to depopulate Africa.
Although the state later resumed the campaign after clinical trials proved the vaccine safe, Kano had already infected other countries in the region that had been considered polio-free.
Traditional and religious leaders are involved in the latest vaccination campaign spearheaded by UNICEF, the World Health Organisation and the US Center for Diseases Control.
Apart from Nigeria, the three other countries where polio is endemic are Afghanistan, India and Pakistan.
The disease is transmitted via contaminated food, water and faeces.