This time two years ago, it must have felt as though the long international campaign to eradicate polio—launched in 1988 and decades past its hoped-for end date—was at last nearing its goal. There were only 17 cases of naturally occurring polio in the world in 2017, half the number from the year before and incomprehensibly fewer than the 350,000 cases that occurred annually when the campaign began.
The picture looks different now. The count for 2019 won’t be concluded until next year, but so far this year there have been 117 cases of naturally occurring polio. And in a galling development, there have been an additional 216 cases of what is called “vaccine-derived polio”—an accidental byproduct of the eradication campaign, brought into being by the campaign’s own vaccines.
In other words, there now exist more cases of polio paralysis caused by vaccines than there are cases caused by the original wild virus. It’s a stunning setback for the hard-fought program, overshadowing the news in October that its relentless rounds of vaccinations have eradicated two of the world’s three wild strains of virus. The campaign now finds itself mired in an asymptote of never quite getting there—a new phase in the fight that may be the most dangerous of all.To step back for some history: Polio causes paralysis, of course (Franklin Delano Roosevelt, who concealed his wheelchair use while in the White House, might be its most famous historical victim), but it has been prevented by vaccination since 1955. By two vaccines, actually: an injected one that uses killed virus to evoke an immune response, and an oral vaccine that uses weakened live viruses. Both of these originally contained cocktails of all three polio strains.
The reason two vaccines exist stems from a long-ago scientific rivalry between Jonas Salk, who developed the first, injectable vaccine, and Albert Sabin, who produced the live-virus vaccine a few years later. But the reason both are still in use comes down to economics, as well as to calculations of probability.The injectable vaccine confers durable immunity quickly: Children are 95 percent protected by their second shot at 4 months old, and completely protected by their third shot anywhere between 6 and 18 months. (A fourth booster shot comes before entering school.) But this version is more expensive to produce than Sabin’s, and its administration requires both a trained professional to give the shot and a system for safely collecting used needles afterward—factors that confine its use to industrialized countries with big health budgets and plenty of health workers.
The oral vaccine is relatively cheap to make and easy to give. Administration requires just dripping the liquid vaccine into a child’s mouth, which anyone who has received a brief coaching can manage it. That has made it the mainstay of the worldwide campaign, administered to millions of children by millions of members of Rotary International, which shares leadership of the campaign with the World Health Organization, the Centers for Disease Control and Prevention, and the Gates Foundation.
The oral vaccine also confers immunity in a less predictable manner. Under ideal conditions, four doses before a child is 4 months old will do the trick. Unfortunately, most of the places where polio has persisted longest don’t offer ideal conditions. When poverty or politics or civil unrest keep adequate doses from arriving, children who haven’t received a sufficient number remain vulnerable. So do any babies born into a community since the last time the vaccinators came around.
When that happens, polio bounces back. That is the situation in Pakistan and Afghanistan, the two countries where wild polio has never stopped circulating. They collectively have 117 cases this year; they had only 33 in all of 2018.