Remember the promise of normalcy by July Fourth? How can it be that now — after Thanksgiving, after so much sacrifice and waiting, after the arrival of vaccines, after months of sweaty masks, distancing and isolation — that another wave of pandemic infection is upon us? In Europe, lockdowns are returning. In some hot spots such as Minnesota and Michigan, hospital wards are again overflowing. A worrying new variant is raising alarms. What happened?

It’s not all bad. The vaccination campaign has put at least one dose in the arms of more than 231 million Americans; those over 65 years old are 86.1 percent fully vaccinated, and the rollout is now reaching the youngest, too. Schools went back to in-person learning this fall, mostly without massive disruption. Despite angry outbursts, it was possible to find resilience and patience everywhere. Face masks have become commonplace; health-care systems and workers fought to save lives amid immense stress and exhaustion; the economy rebounded.

But the pandemic is the unwelcome guest who won’t leave. The delta variant, for reasons still unclear, surges at different places over time. A few months ago, it was rampaging in Florida and the South; now it is in the Upper Midwest. Delta’s behavior is hard to figure. It can set off a precipitous surge, then decline almost as suddenly, as happened in India. Or it can zoom up to a plateau, and stay there, as in Britain. “It is not fitting into a neat bow-tied package of seasonality and predictability,” says epidemiologist Michael Osterholm of the University of Minnesota.