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Two years after Margaret Chan revamped the fight against polio with an "intensified" programme, the disease remains entrenched in Afghanistan, India, Nigeria and Pakistan.

But the WHO and the rest of the world refuse to give up, subscribing to the mantra "Polio has to succeed" — an assertion reflected in the US$635 million polio eradication initiatives received last month alone.

While polio eradication faces operational challenges, the battle is also being fought on biological fronts.

There was a time when experts insisted that research into a new polio vaccine would never be necessary, as the disease would disappear before one could be ready.

But there is a risk that the standard live oral polio vaccine (OPV) could revert to an active form and set off an epidemic — so long as OPV is in use, eradication cannot be achieved.

So experts have begun to look elsewhere. Some scientists are championing the use of inactivated polio vaccine (IPV), but it's expensive, requires injection and its effectiveness isn't proven in tropical settings. The WHO is now looking into ways to make the vaccine go further, and more efficient delivery methods.

Another issue is the type of IPV to use. The standard Salk IPV uses wild virus — posing a risk of accidental release — while the new Sabin IPV, which could be available in 5­8 years, uses a less infectious strain.

Bruce Aylward, director of the initiative, insists eradication is closer to reality than ever before. "We won't let up," he insists. "We still have very long sleeves and lots of tricks up them if we need them."

Link to feature article in Science

Link to vaccines article in Science