Should vaccinated people worry about long Covid?

Over the past few months, experts and officials have tried to prepare the world for a future in which Covid-19 is here to stay. They predict the vaccines will by and large defang the virus. There will still be a few cases of serious illness and death, but the coronavirus will be reduced to the level of a seasonal flu — a disease we’d be much better off without, but mild enough we won’t shut down society to fight it.

But this optimistic vision has always left open a big question:...