DAKAR, 26 April 2010 (IRIN) - The second three-day round of a synchronized campaign to vaccinate 77 million children against polio in 16 West African countries is now underway.

The wild poliovirus has been reported in eight countries in the region in the past six months: Burkina Faso, Chad, Guinea, Liberia, Mali, Mauritania, Senegal and Sierra Leone, according to the World Health Organization (WHO) and UN Children's Fund (UNICEF).

In this second round immunizers will return to areas targeted in the first vaccination cycle - which took place on 6-9 March - where 10 percent or more children were missed.

"It takes a while to identify where the gaps [in rounds] are," WHO spokesperson Rod Curtis, told IRIN, emphasizing that agencies are getting better at it. "Monitoring gaps is far more efficient than it was several years ago ... With independent monitoring we can now identify the gaps within the round itself and return the next day to target children who weren't present."

Prior to 2008, most West African countries had been declared polio-free, until the disease spread out of Nigeria, for the second time in five years.

Nigeria is one of four polio-endemic countries, including Afghanistan, India, and Pakistan.

However, polio levels in Nigeria have plummeted, with just two cases to date in 2010 from 193 in the same period in 2009 - evidence of the impact the involvement of religious and traditional leaders has had in the fight, said Curtis.

All 16 countries involved in the campaign are working closely together as a bloc, Curtis said. "We are very hopeful about eliminating polio in this region if we continue the group effort...the spirit shown by the countries involved is fantastic...if we can complete two to three high-quality rounds, we should be able to raise the immunity level to stop transmission."

Agencies will be able to judge the success of these rounds six months after the campaigns end – in January 2011, says WHO.