A mild US flu season is waning, but is it really over?

"The question we're asking ourselves now is: 'Is this it, or is there more to come?'" said Lynnette Brammer of the Centers for Disease Control and Prevention.

COVID-19 cases have been falling, leading to a decline in mask wearing and behaviors that may have been keeping flu down this winter. As people are less cautious, flu or other respiratory viruses can surge, Brammer said.

Indeed, some indicators of flu activity have inched up the last couple of weeks: a count of flu-related hospitalizations and the percent of specimens from patients with respiratory illnesses that test positive for flu.

Limited data on who is testing positive for flu suggest about two-thirds are kids and young adults. Kids have driven flu's spread in past years, so "it's quite possible we could see continued increases," Brammer said.

Dr. Angela Branche, a University of Rochester infectious diseases specialist, called the flu season unusual.

"I don't have any (flu) cases in my practice this week," she said recently. Normally, doctors in Rochester would be diagnosing 50 to 100 flu cases a day around this time of year.

It seems like the current flu season is "easing to the finish line," said Dr. William Schaffner, a Vanderbilt University infectious diseases expert. But viruses can be unpredictable.

"As the flu-ologists like to say, 'if you've seen one flu season, you've seen one flu season,'" he said.

Last winter's flu season was virtually non-existent. Experts credit mask wearing, social distancing, school closures and other measures to prevent the spread of COVID-19.

Some doctors were nervous about how things would go this winter, wondering if last year's lull would cause flu immunity to wane. Also, fewer children and adults got flu shots this year, according to preliminary CDC data.

The worry seemed to be legitimized by an early November flu outbreak at the University of Michigan in Ann Arbor, where more than 700 cases were reported. The illnesses were caused by a certain version of flu — called Type A H3N2 — that traditionally leads to more hospitalizations and deaths, particularly among the elderly. Worse, many of the infected kids were vaccinated, and investigators concluded the shots offered low levels of protection.

That strain later became the main cause of flu illnesses across the country. But this season has nevertheless turned out to be tame.

That was a surprise, said Dr. Edward Belongia, a flu expert at the Wisconsin-based Marshfield Clinic Research Institute.

"We have occasionally seen other very mild flu seasons, but not where H3N2 is the dominant strain. That's what really makes it odd," he said.

The season peaked in December, just as COVID-19 cases surged, driven by the more transmissible omicron variant, Branche observed. Flu cases dropped as more people masked up and took other steps to prevent coronavirus from spreading, she noted.

Even at its height, the flu season was not nearly as bad as some of the pre-pandemic flu seasons driven by H3N2 strains. Experts aren't sure why.

Some wonder whether the coronavirus essentially muscled aside flu and other bugs. Scientists say they don't fully understand the mechanism behind that.

Of course, a highly effective vaccine would help lessen the severity of a flu season. But researchers say the flu strain that's been circulating is a mismatch for this year's vaccine.

The CDC has not yet released estimates of the current vaccine's effectiveness but it is expected to do so next week.

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