According to a study published this week in the New England Journal of Medicine, researchers report that a polio vaccine developed years ago is much more effective at protecting children against the paralytic disease than a newer formulation.  Researchers say nowhere is this more evident than Nigeria, which harbors the lion's share of global polio cases. VOA's Jessica Berman reports.

In 2003, while the rest of the world had become virtually polio-free thanks to vaccination, Nigeria became a hotbed of the paralytic disease when it halted its immunization program for one year, with the polio rate soaring to 82 percent of all global cases.

In an effort to bring the situation under control, the Nigerian government began an eradication campaign in 2006 that included two different types of vaccine.

The most widely used vaccine worldwide is a one that contains live, weakened virus to stimulate the body's immune system to fight the disease.  Known as an OPV vaccine, this polio vaccine can be either trivalent or monovalent.

A trivalent vaccine confers protection against the three types of polio.  A monovalent vaccine, which targets a single disease-causing type, had been used for many years before the trivalent vaccine was developed.

In a study of both types of vaccine in Nigeria, researchers compared the two drugs to see which was more effective in preventing the disease in thousands of children between 2001 and 2007.

Researchers with England's Imperial College in London and the World Health Organization concluded the older, monovalent vaccine is four times more effective than the trivalent in protecting children against polio.

Senior author Nicholas Grassly, an infectious diseases specialist at Imperial College, says the three polio or sero-types in the trivalent vaccine weaken its effectiveness against the most common form of the disease, type one polio.

"These three serotypes, they provide immunity, but they can interfere with one another in the gut," said Nicholas Grassly.

Researchers found that a child receiving a monovalent type one vaccine has a 67 percent reduced risk of becoming infected with polio compared to sixteen percent risk of infection for the standard trivalent vaccine.

Vaccine scientist Ellie Ehrenfeld at the US National Institute of Allergy and Infectious Diseases says the findings demonstrate a monovalent vaccine is a good tool in the fight against paralytic polio.

But Ehrenfeld thinks the safer and more expensive polio vaccine used in the West called IPV ought to be available around the globe.

She believes IPV, which is made from the killed polio virus, will control the disease once eradication has been achieved.

"I do think and I feel pretty strongly that when you have only one tool in your tool box which in this case is OPV, especially when there exists another tool that is IPV that it is a mistake to put all you eggs in one basket," Ehrenfeld said.

Ehrenfeld's comments were published in the New England Journal of Medicine.