New research suggests a one-two punch could help battle polio in some of the world's most remote and conflict-torn regions: Giving a single vaccine shot to children who've already swallowed drops of an oral polio vaccine greatly boosted their immunity.

The World Health Organisation officials said the combination strategy already is starting to be used in mass vaccination campaigns in some hard-hit areas and is being introduced for routine immunizations in developing countries.

"It could play a major role in completing the job of polio eradication once and for all," said Dr. Hamid Jafari, WHO's director of polio operations, who led the study published today in the journal Science.

Oral polio vaccine has played a critical role in the nearly three-decade effort to eradicate the paralyzing disease, as health workers have gone house-to-house, to refugee camps and to roadside checkpoints delivering the drops.

The number of countries where polio regularly circulates dropped from 125 in 1988 to just three as of last year -- Pakistan, Nigeria and Afghanistan.

But with travel, the threat is re-emerging in countries previously free of the highly contagious virus. The WHO in May declared an international public health emergency, citing outbreaks in at least 10 countries.

Particularly of concern were Syria, Somalia and Iraq, where violence has complicated efforts to contain new cases. Which vaccine to use in the eradication push has long been controversial.

They each have different strengths. The US and other wealthy countries have switched back to using only injected polio vaccine, which is made of "inactivated" or killed virus, for routine childhood immunizations after eradicating the disease within their borders.

That's because the oral vaccine contains weakened live virus that children can shed in their stools, which on very rare occasions can trigger a vaccine-caused case of polio.

The study involved nearly 1,000 children, from babies to 10-year-olds, in northern India in 2011, the last year that country reported a case of polio.

A similar study in 450 children in southern India last year reached the same conclusion, researchers reported in The Lancet last month.

And last December, Kenya put the strategy to its first real-world test.