In the incredibly ambitious, multibillion-dollar effort to wipe polio off the face of the planet, there's currently good news and bad news.

The good news, says Michel Zaffran, who runs the World Health Organization's global polio eradication program, is that there's hardly any polio left.

"When we started back in 1988," Zaffran says, "we had cases in 125 countries and 300,000 cases every year."

Last year what WHO refers to as "wild" polio was found only in two countries — Pakistan and Afghanistan. And it crippled fewer than a hundred kids. Wild polio is the traditional virus as opposed to "vaccine-derived" polio — which we'll explain in a minute.

Today — in what WHO is celebrating as a major milestone in the global effort to wipe out polio — the Type 3 strain of the virus has been officially declared eradicated.

There are three strains of traditional "wild" polio. They all cause the same disease but they each required a different vaccine.

Type 2 was eradicated first. The last recorded wild Type 2 case occurred India in 1999. Type 3 was snuffed out next with the last case found in northern Nigeria in 2012.

Today's announcement means Zaffran and his colleagues now have to contend with only the Type 1 strain of traditional polio.

"So this is significant progress," Zaffran says. "It's slow progress, unfortunately. We'd like to have stopped all wild polio viruses, but it is a very significant accomplishment."

And now for the bad news. It comes on two fronts.

First, wiping out polio entirely in Afghanistan and Pakistan is proving incredibly difficult. The number of cases in Pakistan has spiked dramatically from eight cases in 2018 to 76 so far this year. In Afghanistan this April the Taliban blocked WHO April from vaccinating kids in parts of the country it controls. Last month the Taliban lifted that blanket ban but issued a new order that only allows the polio vaccine to be given in health centers. In eradication efforts the vaccine is traditionally given through door-to-door campaigns. Zaffran says vaccinating exclusively in clinics will never get immunization rates up to the 95% level needed to stop transmission of the virus.

"In Afghanistan we've had only 19 cases so far this year but we've accumulated a large number of unvaccinated" due to the ban, Zaffran says. "So it's likely that there will be many more cases before the end of the year."

The other big problem facing the polio eradication program are ghost viruses linked primarily to the old Type-2 vaccine.

The oral polio vaccine used in lower income countries in most parts of the world contains live virus as opposed to the killed virus used in the injectable vaccine administered in the U.S. and Europe. But injections are problematic.

"It's a lot easier to give kids drops in their mouth than to inject them," says Vincent Racaniello, a virologist at Columbia University. "You have to have people who know how to give injections and so forth. So this is a tough problem."

The point of the oral vaccine is that it has been weakened in the laboratory so that the virus will provoke an immune response without causing the actual disease. According to WHO, the risk of contracting polio from the vaccine is very rare — approximately 1 in every 2.5 million doses administered.

But as billions of doses of vaccine were doled out around the world, some of them started to do what viruses do and spread from one person to another. In places where there's poor sanitation, the vaccine is excreted into sewage, ends up in someone else's drinking water and can survive for months if not years.

At first this was fine because the virus wasn't harmful. It was still very similar to the weakened version in the vaccine. But slowly some of that vaccine-derived virus reverted to a virus just as dangerous as wild polio.

Cases of vaccine derived polio have been steadily on the rise (if you don't count a spike in 2009). Currently there are 12 vaccine-derived polio outbreaks in Africa. Last month another was declared in the Philippines. Prior to that, the Philippines hadn't had a polio case since 1993.

Racaniello, the virologist at Columbia, says there are now more outbreaks of vaccine-derived polio than of the original disease.

"It's actually crazy because we're vaccinating now against the vaccine in most parts of the world," Racaniello says, "not against wild polio, which is confined to Pakistan and Afghanistan."

As Racaniello notes, wild polio virus simply isn't present in most parts of the world anymore. But because of the ongoing need to vaccinate kids against the disease, vaccine-derived polio viruses are all over the place.

WHO is well aware of this problem and is trying to solve it. After Type 2 polio was declared eradicated in 2016, the Type-2 vaccine was taken out of the oral polio drops.

But that has caused another problem. Rogue vaccine-related strains of Type-2 polio continue to pop up, and young, recently vaccinated kids aren't immune to them. With genetic sequencing, scientists can tell that all the Type-2 outbreaks around the world are from viruses that started out as vaccine.

There's also been a push to get more countries to use the injectable, killed vaccine. That vaccine protects against all three types of polio and because it's not a live vaccine, it is physically incapable of reverting to a virus that would cause the disease.

"But that's turned out to be problematic because there's been a production shortage up until not too long ago," Racaniello says.

The other tough problem in the fight to vanquish polio is keeping up the funding and political will to vaccinate hundreds of millions of kids around the world against polio when there are only a handful of cases each year. But Racaniello says the world shouldn't back off now.

"I would just hate it if we gave up and then polio returned in big numbers," he says. "That would be so sad because it's totally preventable. And kids don't have to be paralyzed — at least not by polio anymore."

When the polio eradication initiative was first launched in 1988 the goal was to get rid of the disease by the year 2000. Obviously that deadline wasn't met. The new official target is 2023. Racaniello says it might even take a bit longer than that.

"I think it can be done," he says. But he adds that it could be another decade before the final polio virus is eradicated.

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