Achieving a worldwide elimination of polio, mostly conquered in developed countries long ago, will take a combination of the two available forms of a vaccine, researchers say.

Best results would come from giving children around the world both the inexpensive, easy-to-administer Sabin oral version along with at least a single dose of the injected Salk inactivated vaccine predominantly utilized in developed nations, they say.

The World Health Organization, responding to a study to that effect published in the journal Science, is urging countries exclusively administering the oral vaccine to children and infants start ensuring they also receive at least one dose of the Salk vaccine.

Although considered a disease of the past in the West, polio still resists eradication in parts of Africa and Asia.

In China and Syria, two countries in which the disease has been thought of as defeated, there have been outbreaks in recent years.

That has led the WHO to declare the observed spread of the virus a public health emergency and international concern.

The available prevention -- the inactivated vaccine developed in 1952 by Jonas Salk and the oral version created by Albert Sabin around a decade later -- are both effective at giving a person immunity with as few as three doses.

But there has been an ongoing debate among immunologists and public health officials over which can offer the highest immunity levels and best contain a spread of the virus.

There is also the problem of vaccination campaigns in many countries being disrupted or compromised by mass migrations, conflict or government corruption.

While neither vaccine is completely ideal, the new study suggests, together they can achieve the best results in working toward both goals.

Even in areas where a rigorous program of administering the oral vaccine is carried out, augmenting that with the injectable inactivated vaccine can improve the situation, says research leader Hamid Jafari, WHO's director of polio operations and research.

"The inactivated polio virus vaccine is becoming an important tool in preventing international spread of polio," he says.

In an outbreak of polio, even people who've received the recommended doses of the oral vaccine, who might be infected but have not become ill, should be given an additional injection of the inactivated vaccine to limit the amount of virus they shed and spread to others, he says.

The WHO research results "revolutionize our thinking about how to use polio vaccines optimally," Jafari says.