This winter’s Covid wave in the United States has been the gentlest to date, in a welcome reprieve.

According to wastewater data aggregated by the Centers for Disease Control and Prevention, not only was there less Covid circulating over the holidays than in previous years, but there was also less virus in the wastewater than in all the summer waves the program has tracked.

The Covid hospitalization rate stayed around half of what it was last year, and deaths fell too. In late December, around 600 people were dying each week. Last winter at that time, it was around 2,000. (During the Omicron surge at the end of 2021, weekly deaths were topping 10,000.)

Although wastewater levels can’t tell us how many individual cases of Covid there are, the recent data reflects a significant lull in the virus’s five-year assault.

“This is definitely the mildest Covid winter,” said Michael Mina, an epidemiologist and chief science officer for eMed. “In terms of hospitalizations, in terms of spread.”

A new low

One possible reason for the lull is that the population is still carrying some immunity from a large, later-than-usual summer surge, said Peter Chin-Hong, an infectious disease specialist at the University of California, San Francisco. This year’s vaccine was also a good match for the circulating variant, and more people got it this year than last, according to C.D.C. data.

The virus also didn’t acquire the kind of mutations after the summer wave that would have allowed for significantly faster transmission or greater sickness, epidemiologists said.

That’s not unexpected several years into a new virus, said Aubree Gordon, an epidemiologist at the University of Michigan.

“You have two or three years of it being really bad,” she said. “Usually the first year is the worst — as far as incidence rates and severity goes — and then it settles out.”

Epidemiologists don’t know yet what a “baseline” Covid wave will look like, and there’s no guarantee that each winter will be milder than the last. But the chances of a new variant that can cause significant harm are much lower now, Mr. Mina said.

“Should we expect the variants to start to decline, in terms of how quickly they’re rising, and how aggressively?” he said. “The short answer is yes. The virus has grown up.”

Americans’ immune systems have become very familiar with the virus, said Mr. Mina, through vaccination and prior infections, and on average are more capable of recognizing and attacking it. That means we might have a lower viral load when we become ill, he said, or clear the virus faster, getting less sick and infecting fewer people in the process. Fewer infections also give the virus fewer opportunities to mutate.

Still other ways to get sick

That said, if it feels as if almost everyone you know has gotten sick this winter (or still is), you’re not wrong: It’s been another tough season for other respiratory viruses.

At its peak, the weekly flu hospitalization rate this year surpassed last winter’s high rate; hospitalizations for respiratory syncytial virus (R.S.V.) have similarly mirrored last year. (Norovirus, though not respiratory, is also notably high this year.)

Flu and Covid have had roughly the same death toll so far this season — around 8,000 to 9,000 people as of mid-January, according to C.D.C. estimates. Covid deaths since the start of last summer have totaled around 25,000. (Though getting one virus can theoretically lower an individual’s risk of getting another for a short time, it’s still very possible for multiple viruses to surge at once.)

The comparison with flu is useful because, like flu, Covid is here to stay. As with flu, there’ll be better and worse seasons. It might turn out that this winter was on the low side of our new baseline, Professor Gordon said.

But unlike with flu, there will probably be more waves outside of winter. While the timing of Covid’s winter surge has been relatively consistent — peaking in early January each year — its other waves have yet to fall into a clear pattern. A mild surge during the winter holidays could mean a worse one later this year, possibly even later this winter. And for people who are at higher risk, that will continue to translate into severe illness and death, as well as new cases of long Covid.

“There might be some good times, some bad times,” Dr. Chin-Hong said. “So whether or not we’ll get something later on? We have to have humility.”

But for now, there’s a measure of relief for Americans, as well as for the experts who’ve tracked the virus for five long years. “If I never saw a crazy variant for the rest of my life,” he said, “I’d be so excited.”