The world is still battling polio. What that warning means for COVID-19.

One of the largest and costliest global vaccination efforts ever, meant to rid the world of polio, was on the brink of achieving its goal when coronavirus showed up. Health workers had to overcome a flood of conspiracy theories and other misinformation, reluctance grounded in religious reasons, and additional hurdles to eliminate polio everywhere except for portions of Afghanistan and Pakistan. Their success provided optimism that a similar offensive would help conquer the emerging health threat from COVID-19, outbreaks of which track similarly to historical waves of polio. At the same time, the shift in focus to the pandemic reversed many hard-fought gains against polio in those last two battlegrounds. The Global Polio Eradication Initiative noted a $400 million shortfall in its 2020–21 budget as resources were diverted to fight the new health catastrophe. Lockdowns forced teams conducting door-to-door vaccination sweeps to stop, allowing polio to stage a comeback. Instead of being detecting in 13% of sewer samples in a few hot spots in early 2018, samples extracted late last year reflect prevalence of 74% across Pakistan. When vaccination teams resumed activity in the fall, they faced many of the same barriers as before, with the added challenge of a mutation in the virus. Like polio, health authorities warn that COVID-19 will not be brought under control until vaccines are distributed across the world with near-universal uptake.