US flu activity falls for second week, but experts warn season is not over

Despite the back-to-back declines, infectious disease specialists said the season is far from over. This year’s flu activity has been dominated by an A(H3N2) strain that historically drives severe outcomes in older adults — and roughly 90 percent of the H3N2 infections analyzed this season belong to a new strain that was not included in this year’s flu shots.

U.S. flu activity declined for a second consecutive week, with the number of states reporting high flu activity falling from 44 to 36, according to Centers for Disease Control and Prevention data released Friday covering activity through the week ending Jan. 11. Flu hospitalizations recorded a large drop, and medical office visits for flu-like illness also fell. COVID-19 and RSV have not recorded large surges this winter.

Despite the back-to-back declines, the CDC is characterizing the current respiratory virus season only as “moderate,” and infectious disease specialists cautioned that the trend does not mean the season is winding down. Second surges in flu activity frequently follow the winter holiday period.

“We’ve had other seasons where we’ve had a peak, it’s gone down, but we’ve nonetheless had a prolonged season,” said Dr. William Schaffner, an infectious diseases specialist at Vanderbilt University. “Most of us are crossing our fingers. But I don’t think we can rely on the concept that flu is abating very early this year.”

A mismatched strain amplifies the concern

This season has been dominated by influenza A(H3N2), a strain that historically causes more hospitalizations and deaths among older adults than other flu strains. The concern is compounded by a vaccine mismatch: about 90 percent of the H3N2 infections analyzed this season belong to a new strain that differs from the version included in this year’s flu shots.

Season toll so far

The CDC estimates at least 18 million flu illnesses, 230,000 hospitalizations, and approximately 9,300 deaths since the season began. At least 32 children have died. Of children whose vaccination status is known, 90 percent were not fully vaccinated against flu.