The United States may be headed into a bad flu season, according to figures recently released by the Centers for Disease Control and Prevention.

As of the last week of December, “widespread” flu activity was reported by health departments in 46 states. More ominously, a second measure — the percentage of patients with flu symptoms visiting medical clinics — shot up almost to the peak reached at the height of the 2017-18 flu season, which was the most severe in a decade.

About 61,000 Americans died of flu that season, the C.D.C. said. (The original estimate of 79,000 was revised downward last year; the agency said the number changed as more death certificate information became available.)

This year’s flu vaccine may not be particularly effective against the strain of the virus now widespread in the United States, experts said. But even so, it’s worth getting the shot: people who are vaccinated fare better if struck by the flu than those who are not.