The flu isn’t done yet, as new data suggests U.S. infections are rebounding

A pharmacist prepares to administer a dose of seasonal flu vaccine to a patient at a CVS Pharmacy in Miami, Tuesday, Sept. 9, 2025. (AP Photo/Rebecca Blackwell)

(CNN) — Flu activity has ticked back up slightly in the U.S., thanks to a very small increase in infections caused by influenza B — viruses distinct from the new subclade K strain that’s been the biggest player this flu season so far.

“We are seeing a second peak, a second rebound from flu,” said Dr. Caitlin Rivers, an epidemiologist who writes about infectious disease trends in her Force of Infection Substack. “It’s concentrated in school-age children,” Rivers said, meaning those 5 through 17.

“Children and kids are always the first to register new trends. So I do think that the increase will hit adults as well, but for now, it’s really concentrated in kids.”

Rivers said it’s similar to the pattern the U.S. saw last year, when flu climbed to a second peak in late February after initially falling.

The winter storm that affected about half the U.S. last week may be playing a role in current trends, she added.

New data reported Friday by the U.S. Centers for Disease Control and Prevention shows that B strains are edging up, increasing by less than a full percentage point over where they were the week prior, while flu A activity is holding steady.

Whether this is the start of a spring wave of influenza B remains to be seen, Rivers added.

Influenza B strains are a whole different family of viruses. Even if you’ve had the flu once this season, it’s possible to get it again if you catch a different strain. This year’s flu vaccine protects against two A strains and one B strain, so the shots should offer some protection against severe illness and hospitalization.

Both flu A and B generally cause the same symptoms, such as a high fever, sore throat, cough, body aches and fatigue, and can be treated with antivirals.

Overall, flu activity continues to be brisk, driven by the recently emerged subclade K, an influenza A strain from the H3N2 lineage. Years when H3N2 strains dominate tend to be more severe.

Among lab test results reported to the CDC during the week ending January 24, 87% involved A strains of the flu, while about 13% were from B strains.

The CDC’s national map continues to show a high burden of influenza; 29 states are reporting high or very high flu activity, while 10 states are reporting moderate activity.

Eight more flu deaths in kids were reported last week, bringing the season’s total to 52.

Among flu, Covid-19 and respiratory syncytial virus, or RSV, flu continues to be the dominant respiratory infection, though Covid and RSV activity are increasing.

Brenda Goodman, CNN