As the global effort to eradicate polio gets tantalizing close to its goal, the program is running in to new challenges.

One of the biggest obstacles this year is the proliferation of so-called "vaccine-derived" polio outbreaks.

Conventional polio caused by the traditional form of the disease is now only occurring in two countries in the world — Afghanistan and Pakistan. The World Health Organization calls this form "wild" polio and there've been roughly 100 cases so far this year. This is a tiny number compared to the 350,000 cases that occurred globally before the Global Polio Eradication Initiative was launched in 1988.

But what's troubling now is that there are currently more kids being paralyzed by cases of vaccine-derived polio than by the original "wild" variety.

"We have seen a lot more countries impacted this year than last year," says Dr. John Vertefeuille, the head of polio eradication at the U.S. Centers for Disease Control and Prevention.

There've been outbreaks this year in the Philippines, China, Myanmar, Pakistan and a half a dozen African countries.

"Because of the rising number of individual outbreaks," Vertefeuille says. "The CDC has taken a decision to do a surge staffing effort focusing on Africa."

He says the CDC is in the midst of sending up to 100 additional personnel to Africa to help track, manage and wipeout vaccine-derived polio outbreaks over a six-month period.

Currently around the world, all children are supposed to get vaccinated against polio. In the U.S. and other wealthy nations, kids get 4 injections of inactivated polio vaccine spread out over their first 7 years of life. This injectable vaccine does not contain live virus, so it does not and cannot cause vaccine-derived polio. In lower income countries with weaker health systems, an oral vaccine containing a live but weakened version of the polio virus is used. It's cheap and easy to administer through a few drops in a child's mouth.

In the early days of polio eradication, this live oral vaccine also had the added benefit that it could circulate in a community just like the real, wild virus. In places with poor sanitation, wild polio and the virus in the oral vaccine can spread from sewage to drinking water. At first this was great. Kids who hadn't been vaccinated by health workers ended up getting exposed to the oral vaccine and protected against wild polio because of their lousy drinking water supplies. But over time the virus from the oral vaccine starts to regain strength and if it's allowed to circulate long enough, it reverts back to the point that it can cause paralysis just like the original virus.

"We actually do genetic analysis so that we can understand the closest relative of each (virus) detection that we see globally," says Vertefeuille at the CDC. And in these vaccine-derived outbreaks, they can see that the virus that's paralyzing kids is directly linked to the vaccine that was distributed earlier.

Andrew Noymer, an associate professor of public health at the University of California Irvine says the global polio eradication effort has made incredible progress over the last three decades but now it's reached a difficult moment. He compares the current efforts to vaccinate every child against polio to being stuck on a treadmill.

"You have to keep vaccinating all the children so you won't have any paralyzed children," Noymer says, "But the more you vaccinate, the more live virus continues to circulate. And the minute you step off the treadmill, you get some paralyzed kids."

One way to get off that treadmill would be to get rid of the live oral vaccine and switch to the not-live, injectable vaccine used in the United States. The Global Polio Eradication Initiative has tried to move in that direction but there are several challenges. First there just isn't enough supply globally of the injectable vaccine to cover the hundreds of millions of kids in low income countries. Training vaccinators to give injections is much harder than training them to give two drops in to a child's mouth.

Also the vaccines work slightly differently — with the oral version doing a better job of breaking chains of transmission and stopping outbreaks.

The Switch

A few years ago the world looked like it had a chance to get off the virus/vaccine treadmill at least partially. There are three distinct strains of polio and each strain requires a different vaccine to prevent against it.

In 2015, the WHO declared Type-2 wild polio eradicated. But the problem was that the oral polio vaccine still contained live weakened Type-2 virus and while the wild Type-2 was gone, occasionally there continued to be outbreaks of vaccine-derived Type-2 polio.

So the Global Polio Eradication Initiative orchestrated a maneuver that insiders call the "switch". In April 2016, everywhere around the world the old oral polio vaccine that contained all three strains of the virus were to be pulled out of circulation and replaced with a new vaccine. The new oral vaccine would only contain virus to protect against types 1 and 3. Health officials expected that some rogue Type-2 viruses would pop up and they had plans to quickly attack any Type-2 outbreaks that might occur. But they also expected that Type-2 vaccine-derived polio would die out relatively quickly.

"It was it was a bold move," says Noymer, who studies the history of outbreaks. "It was a sign of incredible progress. But it was a gamble and it didn't pay off."

Now, three years after Type-2 polio was declared eradicated, the most problematic form of polio is vaccine derived Type-2.

That's because in order to respond to those predicted vaccine-derived Type-2 outbreaks, health officials inoculate kids near the outbreak with a specially stockpiled vaccine that protects only against Type-2. But there can be spillover from these mop-up campaigns, which gives the virus another chance to get into the environment, circulate and regain strength in communities. People who were never immunized end up getting exposed, sparking another outbreak.

Since the switch in April 2016, health officials have distributed roughly 300 million doses of this emergency vaccine to combat outbreaks. By doing so they've put hundreds of millions of viruses in to the environment that kids are no longer routinely being vaccinated against.

The switch seemed like a good idea at the time, but it turned out to be a blunder, says Noymer. Public health drives and particularly eradication campaigns can be hard to sustain. In the mid-20th century there was a lot of talk about "donor fatigue" and whether efforts to wipe out malaria and other tropical diseases would get the funding needed to finish the job. But he says the situation with polio now is different.

"The danger in the 21st century, in the age of social media and the age of anti-vaxxers is not that the donors will lose interest and be fatigued because they've shown to be very patient," Noymer says. "But rather that populations will start saying, you know, we're sick of this." He says the real threat could be that parents no longer see any reason to get their kids repeatedly vaccinated against a disease that they don't see anywhere around them. "And then, of course, you get more people who are susceptible to polio if that were to come to pass."

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