Other way around, as I understand it: once the vaccine-derived strains have been circulating for a year or so, they become every bit as virulent and dangerous a

Other way around, as I understand it: once the vaccine-derived strains have been circulating for a year or so, they become every bit as virulent and dangerous as the original wild polio and remain that way indefinitely unless they're stamped out using vaccination. The only way we can tell they're vaccine-derived rather than wild at that point is by using genetic sequencing to trace their heritage since they're otherwise exactly like wild polio. If we could somehow ensure that the vaccine strains didn't circulate for so long they wouldn't be able to regain their virulence and we wouldn't have this problem, but that's easier said than done.