This season’s dominant H3N2 strain, he added, poses “a particular threat to older people,” although existing antiviral medications appear effective against it.

Perhaps people grew complacent because last season, despite fears of a “twindemic” that would simultaneously hospitalize both flu and Covid-19 patients, flu infections actually plummeted. “The lowest season ever,” Dr. Schaffner said.

Last winter “we were all at home, all wearing masks,” Dr. Schaffner said. “Children play a major role in spreading the flu, and last year they were all learning remotely. Now, all of that is reversed” — and the C.D.C. reports that outpatient visits for flulike illnesses are climbing sharply, especially in the East and Midwest.

Even if the flu season turns out to be moderate, with returns to school, work and travel, and with the rampaging Omicron variant, this could be the year that both flu and Covid patients swamp struggling hospitals and health systems.

Moreover, researchers are reporting that flu can lead to other health crises, especially in older patients. In 2018, Canadian researchers found a link to heart attacks, for instance.

Using data from Ontario residents (average age 77) who had influenza, “your risk of having a heart attack is six times higher during the seven days after testing positive” than in the year before or afterward, said Jeffrey Kwong, a senior scientist at ICES, a nonprofit research institute in Toronto. Other studies have found similar results.

The cause might be increased stress on the heart from a fever, Dr. Kwong noted; respiratory viruses can also create inflammation. In either case, “the influenza vaccine can actually decrease the risk of hospitalization and death from heart attacks,” he said.