LONDON, (Reuters) - In a huge immunization effort in 150 countries, health teams will on Sunday launch what they hope will be the final push against polio.

Stopping transmission of the contagious viral disease that has infected millions is possible within a year, experts say. And full, official, global eradication could be declared by the end of this decade.

First, however, the vaccine that has successfully fought polio for more than 30 years needs to be switched for one that targets the last few areas of risk.

It won't be easy, or cheap, but the World Health Organization's director of polio eradication, Michel Zaffran, says failure now - when there have only been 12 cases worldwide this year, in Pakistan and Afghanistan - means the virus could spread across borders again.

Success would make polio only the second human disease to be eradicated since smallpox was banished in 1980.

"Taking our foot off the pedal now could mean polio will within a few years spread straight back into large parts of the world and create 100,000 or 200,000 cases," Zaffran told Reuters. "The job has not been done and will not be done until we have fully eradicated the virus."

VACCINE COMPLEXITIES

For the endgame in polio to succeed, a coordinated and complex vaccine switch is crucial.

Until now, many countries have been using a shot that protects against the three types of wild polio virus - type 1, type 2 and type 3 - but type 2 polio transmission has been stopped since 1999, meaning immunizing against it now makes no sense.

In rare cases it also poses a risk that the weakened type 2 virus in the vaccine can seep into circulation and cause "vaccine-derived" polio infections.

So from April 17 to May 1, some 150 countries will engage in a synchronized switch to a bivalent, or two-strain, vaccine that contains no type 2 virus but targets types 1 and 3.

It's a massive undertaking and a major step towards eradication, says Zaffran. "We're entering into uncharted territory. This has never been done before. But there's no going back now."

That's partly because polio vaccine manufacturers - among them France's Sanofi Pasteur - have moved production to the bivalent shot and would find it tricky, costly, and time-consuming to reverse that move.

Anil Dutta, a vaccine expert at British drugmaker GlaxoSmithKline, which also makes polio shots, is looking beyond eradication to 2019 or 2020, when all "live" oral polio vaccines need to be discontinued.

Then the world will switch again, to "inactivated" polio vaccine, or IPV, to further reduce any risk of causing disease through immunization. Scaling up IPV production to meet the needs of the entire world takes years, he warns, and work must start now to avoid potential supply concerns.

HISTORY OF MISSES

But prediction has never been easy in the fight to wipe out polio, and health authorities have missed targets along the way.

The Global Polio Eradication Initiative, launched in 1988, originally aimed to end all transmission of the disease by 2000.

And while there has been a 99 percent reduction in cases worldwide since the GPEI launch, fighting the last 1 percent of polio has been far tougher than expected.

In 2013, the GPEI said the global fight against polio would require $5.5 billion in funding, and more will be needed beyond that to keep a lid on the disease.

The virus, which invades the nervous system and can cause irreversible paralysis within hours, spreads rapidly among children, especially in unsanitary conditions in war-torn regions, refugee camps and areas where healthcare is limited.

In Pakistan and Afghanistan, the last two countries where polio currently remains endemic, conflict and propaganda have hampered progress, and in the past posed risks to others.

The campaign to eliminate polio in Pakistan is fraught with risk, with Islamist militants attacking health teams they accuse of being Western spies. A polio worker was shot and wounded in February and in January a suicide bomber killed 15 people outside a polio eradication center in the city of Quetta.

In 2011, a polio virus from Pakistan re-infected China, which had been polio free for more than a decade.

In 2013, the disease re-emerged in Syria after a 14-year absence, prompting the need for a vast and expensive regional emergency vaccination campaign.

And last year, cases of type 2 vaccine-derived polio posed new threats in Ukraine and Mali.

David Salisbury, an immunization specialist and associate fellow at Britain's Chatham House Centre on Global Health Security, says the last 1 percent is a "very long tail" on a stubborn epidemic.

"The original date for interruption of transmission was 2000. The next target was 2014 and it's currently 2016," Salisbury told Reuters, adding that even with case numbers as low as they are now, "2016 may be optimistic".

Liam Donaldson, head of the Independent Monitoring Board of the GPEI, agrees that celebrating the expected extinction of polio virus "would not just be premature, it would be folly".

"Polio is still out there," he told a meeting in London. "(It) has fought back with a vengeance at every stage of the game. And it's still fighting."

(Reporting by Kate Kelland; editing by Giles Elgood)