In a huge immunisation 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 Organisation’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.”
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 immunising 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 synchronised 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.”
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 per cent reduction in cases worldwide since the GPEI launch, fighting the last one per cent 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 health care 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 centre 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.