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Echoing patterns in prior years, coronavirus infections are slowly ticking up in parts of the country, the harbinger of a possible fall and winter wave. But the numbers remain low for now, and are unlikely to reach the horrific highs seen in previous winters, experts said in interviews.

Infections have been trending upward for about four weeks now, according to data gathered from wastewater monitoring, test positivity rates and hospitalizations and emergency room visits. Taken together, the figures offer researchers and public health officials the first glimpse of the coronavirus as a post-pandemic, seasonal threat, a permanent fixture of the infectious disease landscape.

Wastewater analyses point to the highest increases in the Northeast and the South, followed by the West and Midwest. After hitting a trough at the end of June, hospitalizations are inching upward again, but fortunately very slowly.

Test positivity has risen to 7.6 percent, a level last seen in November 2021, and that summer, just before the Delta variant swept the nation.

“This is the fourth summer now that we see a wave beginning around July, often starting in the South,” said Caitlin Rivers, an epidemiologist at the Johns Hopkins Center for Health Security.

Nearly all Americans have built up multiple layers of immunity following repeated infections, immunizations or both, so the virus is unlikely to cause the harm this winter that was seen in previous seasons.

Still, for older adults, pregnant women and people with weakened immune systems or certain chronic conditions, the virus may yet pose a serious threat.

The number of deaths is the lowest since the pandemic began, and roughly one-tenth of the levels in January. Most virus fatalities now occur in adults older than 75. But the real toll will be apparent only at the end of the year, after the fall and winter’s respiratory blitz, experts said.

“We are in a very different place, but Covid is still a thing,” said Katelyn Jetelina, a public health expert and author of the widely read newsletter, “Your Local Epidemiologist.”

“I think we do the public a disservice by saying that it’s over and let’s move on, because it is going to be disruptive this winter, and it will cause a number of people to die,” she added. “That’s just not acceptable to the public health world, especially since it’s preventable.”

Researchers have been trying to assess how updated Covid vaccines and emerging variants might change the course of the pandemic. By the most pessimistic estimates, if no vaccine were available and the circulating variant dodged most immune defenses, Covid might lead to about 839,000 hospitalizations and around 87,000 deaths nationwide between September and April.

In the best-case scenario, with people of all ages opting for an updated vaccine and a variant that is susceptible to that vaccine, Covid might cause 484,000 hospitalizations and 45,000 deaths — about the toll of a bad influenza season.

“Based on these projections, Covid is likely to remain in the leading causes of death in the United States for the foreseeable future,” said Justin Lessler, an epidemiologist at the UNC Gillings School of Global Public Health who coordinated the research effort.

The range of estimated deaths would place Covid somewhere between liver disease and diabetes for causes of death. “Even in that most optimistic scenario, we’re getting into the range of mortality that we see for top 10 causes of death in the United States,” Dr. Lessler said.

Experts worry in particular about the confluence of Covid with respiratory syncytial virus, influenza and other pathogens. Many hospitals buckled under the weight of the so-called tripledemic of Covid, flu and R.S.V. last year, even though waves of the three infections appeared to be slightly staggered.

R.S.V. peaked in November and caused about twice as many hospitalizations, including among children, as in prepandemic years. The flu peaked in December and may have led to as many as 58,000 deaths.

Covid led to an estimated 50,000 deaths between November and March. It’s unclear whether the viruses will behave similarly this winter or will drift into a new seasonal pattern.

“This fall is something that us epidemiologists are watching with much curiosity,” said Dr. Jetelina. “I think a lot of us are cautiously optimistic that we may start getting a new normal respiratory season.”

Even if the peaks of each viral wave are further apart than they were last year, the health care system may struggle.

“Even before Covid, it was very difficult for health care systems to keep up with the surge of patients,” Dr. Rivers said. “If this is, in fact, what we can expect year over year going forward, I think we’re going to have to adjust the health care system to accommodate that increased load.”

The coronavirus is still a more formidable threat than the other two respiratory infections, Dr. Rivers said.

Unlike flu and R.S.V., which tend to disappear in warmer months, coronavirus infections start to pick up in July and remain high through February. “That’s a good chunk of the year where you’ve got to be on alert,” Dr. Rivers added.

One striking change from previous years is that instead of a single dominant coronavirus variant, there now appear to be a cluster of viral types, all derived from the Omicron branch. The virus is mutating now at a more constant rate, akin to the pace of evolution of the flu virus, Dr. Lessler said.

The vaccine expected this fall is designed to target a variant called XBB.1.5, which was dominant this spring. Even if the vaccine is not a perfect match for the variants circulating in the coming months, it is still likely to prevent severe illnesses and death, if not infections.

Dr. Lessler and his colleagues estimated that vaccinating Americans of all ages could reduce the number of hospitalizations and deaths by about 20 percent.

No researchers foresee a return to the worst days of the pandemic. But some recommend that when the number of cases go up, people consider wearing masks again in crowded indoor spaces, testing when they have symptoms and being mindful of those around them who may be at high risk should they become infected.

“Whether we’re completely out of the pandemic and settled into our seasonal routine, I am going to pencil in the yes,” Dr. Rivers said. “But I’m also prepared to be surprised, because this virus has surprised me before.”

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