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.

The latest jolt came Friday over a new variant, omicron, spreading fast in South Africa and designated a “variant of concern” by the World Health Organization, which said it has a “large number of mutations.” It will take time to determine if the variant is more transmissible than delta, or more virulent, but it is a worrisome development.

In the United States, the pandemic is being fueled by the unvaccinated: 47 million adults and 12 million eligible teenagers. New daily cases nationwide have been on the upswing for three weeks. Michigan, which had as few as 102 new daily cases at one point in the summer, now has a seven-day average of more than 7,000. At Spectrum Health, a system of 14 hospitals and other health-care facilities in western Michigan, 86 percent of the hospitalized covid patients and 90 percent of those in intensive care units are unvaccinated, many with underlying conditions as well.

Increasingly, waning efficacy of the vaccines is giving the virus room to spread. A study of more than 780,000 patients in the Veterans Health Administration from February to October showed vaccine effectiveness fell from 87.9 percent to 48.1 percent. Because older and immune-compromised people were vaccinated first, their immunity is declining first as well, leading to a growing share of hospitalizations, but younger vaccinated age groups will also experience waning immunity as they reach the six-month mark. The phenomenon should compel all adults to get a booster shot.

One thing is certain: Vaccines work. The virus remains with us, but it does not have to kill.