Vaccine-Derived Polio Is On the Rise

An anonymous reader quotes a report from NPR: Is polio making a comeback? The world has spent billions of dollars over the last 15 years in an effort to wipe out the virus through vaccination efforts -- with encouraging results. Rates plunged from an estimated 350,000 cases in 1988 to just several dozen by 2016. But in recent years, polio incidence has started to inch back up. The reason has to do with the type of vaccine used in many parts of the world, primarily in low- and middle-income countries. While the United States and other Western countries inject an inactivated virus that poses no risk of spread and are now polio-free, other countries rely on an oral vaccine. It's cheap, it's easy to administer and two or more doses confer lifelong immunity. But it's made with living, weakened virus. And that poses a problem. Those who've been immunized with live virus can shed it in their stool, which can then spread through sewage in places with poor sanitation. If the virus stays weak, it can even expose the unvaccinated to polio and give them immunity. But if it mutates and regains virulence, someone who isn't vaccinated can become sick with vaccine-derived polio after contact with the contaminated wastewater. And now countries that had previously eradicated polio in Africa, the Middle East and parts of Asia are seeing new outbreaks of vaccine-derived polio. One reason for this rise in cases, say polio experts, is that gaps in immunization in recent years have created more opportunities for the unvaccinated to become infected. "Vaccination campaigns have been certainly affected by the pandemic," says Raul Andino, a virologist at University of California, San Francisco. [...] The composition of the oral vaccine has also been a factor. In 2016, eyeing an uptick in vaccine-derived polio, global health officials altered the composition of the oral vaccine. Previously, the vaccine protected against all three types of wild polio -- the virus that circulates naturally in the environment. Then they withdrew one of those types -- the one that was leading to most of the vaccine-derived cases but whose wild form had been successfully eradicated. Only there was a development that hadn't been anticipated. Vaccine-derived poliovirus of that type was still in circulation from earlier iterations of the oral vaccine -- and now with the reformulated vaccine, increasing numbers of people who were no longer vaccinated against it. So there was further spread. Thankfully, a new kind of vaccine being rolled out is showing promise. "The novel vaccine still contains a weakened version of the virus, but it's been hobbled even further," reports NPR. "The researchers tweaked the virus so that it has to accumulate more mutations to become virulent and has a harder time amassing those mutations. For example, they've altered the polymerase, one of the key enzymes responsible for introducing mutations, reducing its ability to mix and match genes from different viruses."

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