The entire world has 2 weeks to switch over to a new oral polio vaccine. Here’s why.

The world is closer than ever to eradicating polio, the horrible disease that inflicts paralysis on its primarily young victims. It's not far-fetched to say that very soon the world will see its last polio case.

But the tail end of the eradication efforts has been threatened by the very thing that made eradication programs a success: the vaccines themselves, which — in rare instances — can cause small outbreaks.

To combat that risk, starting this week 155 countries are switching over to a slightly safer form of the oral polio vaccine. The new vaccine is a lot like the current one, except that it will no longer immunize against one type of polio (Type 2), which was declared eradicated in 2015. In the next two weeks, the old vaccine will be discontinued completely.

"This represents the largest withdrawal of one vaccine, and associated roll out of another vaccine in history," the Polio Eradication Initiative reports. International health agencies procure billions of doses of the oral vaccine every year.

The swap is no small feat, and it's an important first step toward the complete phase-out of oral vaccines — which in all forms contain a tiny risk — so that eradication can really be achieved.

How the vaccine is being made safer

Photo by Carsten Koall/Getty Images

The World Health Organization's polio eradication program, which began in 1988, has been one of the greatest success stories in global health. At its peak in the middle of the 20th century, polio killed half a million people every year. In 1988, there were more than 125 polio-endemic countries. Now there are two: Pakistan and Afghanistan (where ongoing conflict with the Taliban has made eradication efforts difficult).

This success can be traced back to a tiny vial. In the 1950s, Albert Sabin noticed that not all strains of poliovirus infected monkeys when exposed. He was able to isolate these strains — and prove they wouldn't infect humans — and form them into a swallowable serum that could be easily distributed the world over.

That oral polio vaccine (or OPV, as it's often called) became the favored vaccine of the World Health Organization in its eradication efforts. It's extremely effective at immunizing the gut, where polio takes root in the body. Some 3 billion children have received this vaccine in the past 10 years alone.

Make no mistake: The world is this close to eradicating the disease because of the oral vaccine. But the oral vaccine also comes with a small risk: It can cause new cases of the disease.

The last wild version of Type 2 poliovirus was seen in India in 1999. (the vaccine contains weakened forms of the three main polioviruses — Type 1, Type 2, and Type 3.) But since then, the number of people who contract vaccine-derived Type 2 polio has held steady, as you can see in the chart from the Centers for Disease Control and Prevention below.

CDC

It's also possible for the Type 1 and Type 3 viruses to cause vaccine-derived outbreaks, but these are much rarer. According to the Polio Eradication Initiative, 90 percent of all the vaccine-derived cases of polio in the past 10 years are Type 2.

How can a vaccine cause outbreaks?

All oral polio vaccines contain live viruses, which can mutate back into a virulent form.

"Very quickly after you take the vaccine, [oral-vaccine viruses] revert" to a dangerous form, Vincent Racaniello, a Columbia professor of microbiology and immunology, told me in January. This typically isn't a problem, since by the time they become virulent, a person — and his or her surrounding community — is immune. "But for some reason, one in 1.5 million kids who get the vaccine get paralyzed," he says. That number is tiny, but not insignificant.

And the risk isn't just to those kids unlucky enough to get polio from the vaccine. These vaccine-derived polioviruses also cause new outbreaks in communities that have spotty immunization coverage and poor sanitation.

"It's happened over and over," Racaniello says. "We have to stop using OPV [oral polio vaccine] as soon as possible. As long as we're using it, we're introducing reverted strains into the environment."

A person can shed a vaccine-derived virus for a few weeks. In case one of these Type 2 vaccine-derived strains does start circulating after the vaccine switch (via someone who gotten the old vaccine containing type-2), there's a global stockpile of 500 million doses of a Type 2 vaccine ready to deploy.

Minimizing this risk is exactly why countries are switching over to the new oral vaccine at the same time.

And for further assurance, the Polio Eradication Initiative reports it's "working to increase immunity levels to type 2 poliovirus to a historically high level world-wide."

An end to all oral vaccines is near

Photo by Daniel Berehulak/Getty Images

The World Health Organization will eventually like to phase out all of the oral vaccines — even this new version — as they all contain a tiny risk. Outbreaks of Type 1 or Type 3 vaccine-derived polio are rare, but they can happen.

"Sooner or later you get to the point — in theory you get to the point — where all the [polio] cases in the world are caused by the vaccine," Philip Minor, the head of virology at National Institute for Biological Standards and Control in the United Kingdom, told me in January. "So you are basically using the vaccine to protect against the vaccine. And that is just ludicrous, isn’t it?"

The difference between the number of cases of wild-type polio and the number of vaccine-derived cases is growing very narrow. In 2015, according to the Polio Eradication Initiative, there were 70 cases of polio across the globe from wild strains of the virus. There were 26 cases of vaccine-derived polio.

The World Health Organization reports the oral vaccines will be phased out in a series of steps.

• First is the removal of the Type 2 polio strain from the oral vaccines, which is happening now.
• As this happens, the WHO would like to see one dose of the oral vaccine replaced by an injection of the inactivated vaccine (which has no chance of causing paralysis).
• Finally, all oral vaccines will be (ideally) phased out by 2020, as injected vaccines — which are more expensive and more difficult to administer — become the standard.

This transition is difficult, requiring intense coordination between many countries, global health agencies, and drug manufacturers. But if the world truly wants to eradicate polio — in all forms — it has to be done.

In the past, the risk from the oral vaccines was extremely well worth it. But now that we're on the brink of total eradication, it's unacceptable. This is a good problem to have.

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