For nearly his entire life, a 29-year-old man living in the United Kingdom has had the polio virus living in his gut—where the strain has mutated from the weakened form he received in a vaccine as an infant to a much more virulent strain, according to a study published in PLOS Pathogens. It's a case that NPR sees as "a worrisome new development in the polio end-game." Live Science and the Guardian report the man was given an oral form of the polio vaccine at 5, 7, and 12 months. Unlike the injectable version, the oral vaccine uses a live virus that's typically excised from the child's gut within two months. But the man had an immune disorder that killed his gut's ability to kill the virus. And so it has lingered there, and in his stool: More than 100 samples taken over the past 20 years have returned significant levels of what had become a mutated virus.
As for how potent that virus has become, Live Science reports that it was able to paralyze a mouse but was found to be harmless when tested on the blood of vaccinated humans. Just 73 cases of polio living long term in someone's gut have been found in the past 52 years, and only about 10% of those extended beyond five years. Researchers write that the case "represents by far the longest period of excretion" known from "the only identified individual known to be excreting highly evolved vaccine-derived poliovirus." But researchers believe there may be more individuals like him based on sewage samples taken in countries as varied as Finland and Israel. The implications, per the Guardian: It sees the potential for immunocompromised patients like this man to spur a fresh polio outbreak and observes that "before the disease can be completely wiped out, mutated strains of polio virus derived from vaccines must be tackled." (Meanwhile, the CIA promises to stop using vaccine campaigns as a cover for spying.)