So far this year, only nine polio cases have been reported in the entire world. By this time next year, the World Health Organisation predicts that number will finally reach zero.

"We absolutely need to keep the pressure up, but we think we could reach the point where we have truly interrupted the transmission at the end of the year or the end of the low season [winter] next year," the WHO's polio eradication director Michel Zaffran told The Guardian.

Polio would become only the second human disease we've ever wiped from the planet. And it has taken an incredible global public health collaboration to get here.

Making a bold move to achieve eradication

In the course of just two weeks starting April 17, the entire world is going to do something remarkable: 150 countries are going to switch from one kind of polio vaccine to another.

Fawad Rahmani, 11, has had polio since he was two years old. He lives in Kabul, Afghanistan, one of the few countries where people are still fighting the disease. Credit: Getty Images/Paula Bronstein

They'll change out the trivalent version of the vaccine – one that protects against all three types of the polio virus – to the bivalent version that protects against two.

This is because one of the types in the trivalent version of the vaccine hasn't had any wild cases since 1999.

But the live, oral vaccine can sometimes mutate and reinfect people (more on that below). Eliminating this type from the vaccine will keep that from happening so that when polio is wiped out, it has no chance of making a comeback.

A physical therapist assists two polio-stricken children in this 1963 photo. Credit: CDC (PHIL) / Charles Farmer

The Global Polio Eradication Initiative calls the two-week event "one of the most ambitious globally synchronised projects in the history of vaccines".

This effort is one of many ambitious steps the world had taken to erase the disease from the planet. And in just a year, that dream could become a reality.

On the verge of elimination

In 1952, at the height of the polio epidemic, more than 20,000 Americans got polio. Many were left permanently paralysed. We've come a long way since then. In 2015, there were only 96 cases of polio in the whole world.

A health worker marks the finger of a child after administering a polio vaccine in the northern Indian city of Chandigarh. Credit: Thomson Reuters

Up to this point, we've only ever completely eradicated one human disease: smallpox. The last case occurred in 1977. Guinea worm will likely be eradicated soon as well, since it only had 22 cases in 2015.

This progress to combat polio is incredible, since as recently as 1988, 350,000 people had the debilitating disease, the majority of whom were children.

The Global Polio Eradication Initiative – a partnership between the WHO, the CDC, UNICEF, and Rotary International – sums up the coordinated global effort behind this remarkable decline:

"In 1988, when the [Initiative] began, polio paralysed more than 1,000 children worldwide every day. Since then, more than 2.5 billion children have been immunised against polio thanks to the cooperation of more than 200 countries and 20 million volunteers, backed by an international investment of more than US$11 billion."

The World Health Organisation estimates that 1.5 million children's lives have been saved and 13 million people are able to walk today because of the public health push to eradicate polio.

A girl receives an oral polio vaccine. Credit: CDC Global/Flickr

Not gone yet

Polio is highly infectious. It's spread through human contact, usually through infected stool – a particularly dangerous route of transmission among children who are not yet potty-trained, as well as in areas with poor sanitation systems. People can also catch it by coming into contact with contaminated food or water.

Afghanistan and Pakistan are the only two countries where polio is still spreading naturally. They were responsible for all 74 wild cases last year, and all nine wild cases reported so far this year.

The other 26 cases in 2015 were due to the very rare occurrence of the oral vaccine mutating and causing the virus, which is called "vaccine-derived poliovirus", according to the Global Polio Eradication Initiative.

When someone is immunised, they excrete some of the weakened virus that is used in the oral vaccine. In areas of poor sanitation, that weakened version of the virus can then spread further, providing what the WHO calls "passive immunisation".

But in severely under-immunised populations, that weakened virus can circulate for a year or more, giving it time to mutate and strengthen into a virus that can actually cause paralysis.

These cases are exceedingly rare; there were billions of doses of the oral polio vaccine distributed between 2000 and 2015 and fewer than 760 cases of vaccine-derived poliovirus. "The small risk … pales in significance to the tremendous public health benefits associated with" the vaccine, WHO writes.

Up to this point, we've relied on the oral vaccine because it's more effective at preventing the spread of polio from person-to-person, it's easier to administer, and it costs only 15 cents per dose (compared to $1 for the injected vaccine).

Polio virus. Credit: CDC/Joseph Esponsito

But once no more wild cases are reported, health agencies around the world are going to switch over to the other kind of vaccine, which is injected and contains an inactivated form of the virus that can't mutate back to an infectious state. (This will be the second vaccine switch; the one happening on April 17 is from the trivalent vaccine to the bivalent vaccine, but most of those are oral vaccines for now.)

In February, Walter Orenstein, associate director of Emory University's Vaccine Centre, co-authored a paper on the final steps in the global effort to eradicate polio. He emphasised how imperative it is that we switch over – first from trivalent to bivalent, then from the oral form to the injected one.

Even after no more polio cases of any kind are reported, health officials will have to keep vaccinating children and monitoring the world to make sure the disease stays gone. It took three years after the last case of smallpox before the WHO declared that disease officially eradicated in 1980.

There is a chance that this year could be the last that any child in the world gets polio, marking a major medical and humanitarian achievement.

And our success with polio, Orenstein said in an audio interview published alongside his paper, could lead to the eradication of other diseases.

"To accomplish [eradication], humanity has to unite against a common enemy – in this case the polio viruses. And this hopefully will be what will be achieved in the near future," he said. "And then we can move on to other things that currently afflict the populations around the world."

This article was originally published by Business Insider.

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