In Afghanistan, the problem has been religious unrest. The Taliban imposed overlapping bans on vaccination, both in big events and in house-to-house “mop-up” days afterward. (They are now allowing vaccine to be administered in public buildings, but not in homes.) In Pakistan, the issue is political change. The national election in July 2018 triggered turnover in personnel throughout the bureaucracy, which led to public health officials dropping and picking up tasks without smooth handoffs.

“We have lost basically a year and a half,” says Michel Zaffran, WHO’s director of polio eradication. “The number of children who are susceptible to contracting the disease is increasing very rapidly, and we could actually have explosive outbreaks.”

There’s a further complication. The eradication campaign didn’t choose the oral vaccine for worldwide distribution just for its cost and ease of use. The decision to use the live formula was also influenced by a quirk that made that version especially appealing as a public health tool: the fact that a dose given to one child could actually protect several children.

It works like this. Live poliovirus, both the original wild type and the weakened vaccine version, attaches and replicates in tissues that line the gut. That means anyone infected with polio can pass the virus along in feces. But it also means that a child who has received the weakened vaccine virus can pass the protective variety along instead, making them a secondary source of protection for other children who didn’t receive the drops.

The gain of passing along vaccine virus comes with a risk, though, and this is what has backfired on the polio campaign. Roughly once out of every 2.5 million doses given, the weakened vaccine virus reverts in the gut to the ferocity of wild-type polio, making a vaccinated child a potential source of infection instead of protection.

That probability was acceptable in the early days, when it seemed the campaign would be over quickly. Billions of doses later, the calculation looks different. And it’s been made more complicated still by a decision the eradication campaign made the first time one of the strains of polio virus was eradicated in the wild. In April 2016, those in charge switched from using a three-strain vaccine to a two-strain one. Officials at the time described the switch as a gamble worth taking. The strain that was dropped (called “type 2”) does a better job of attaching to the gut, so its presence in the vaccine could interfere with establishing immunity to the other two, still-circulating strains. With type 2 polio gone from the world, not inducing immunity to it in future cohorts of kids seemed a worthwhile risk.


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In fact, vaccine-strain type 2 persisted in children who were not fully immune, and it rebounded. Most of the vaccine-derived outbreaks now occurring are caused by type 2. The eradication campaign has chased those outbreaks with a supplementary vaccine that contains only type 2, and is also pushing development of a new type 2 vaccine. But the persistence of type 2 outbreaks has brought home the fact that the campaign probably cannot risk refining the vaccine again, to only the type 1 strain of the virus—even though that is the strain that most needs curbing in the world.

Researchers within the campaign acknowledge that vaccine-derived polio is a real risk, but one worth taking overall. “The program has prevented 18 million children from being paralyzed by polio,” says John Vertefeuille, the CDC’s polio eradication branch chief. “To date we have seen less than 1,200 cases of vaccine-derived polio, against a backdrop of delivering billions of doses of vaccine.”

It’s also a risk that should go away someday. Plans call for the last phases of the campaign to roll out one round of the injected vaccine across the world. (It carries no risk of creating vaccine-derived strains because it uses killed viruses and bypasses the gut.) But because the injectable version is costly and limited in supply, it can only be used to seal the achievement, not to tamp down the epidemic’s repeated flare-ups.

Paradoxically, it is the success of the polio campaign that has exposed its fragility. The moment when the end comes into view and a final push is most needed, that turns out to also be the moment when governments find other uses for funding, and when communities grow tired of hauling their kids out for yet another round of drops. The campaign is 31 years old; it has consumed entire careers. It will have to find fresh energy, at a delicate moment, to make it to its goal.

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