As New Flu Variant Spreads in US, Researchers Unsure If More Severe Than Usual

'We’re still in the middle of trying to figure out whether it’s producing worse illness,' one researcher says.
Published: 12/18/2025, 4:21:15 PM EST

Areas across the United States and Europe are seeing an increase in a new influenza variant, researchers say, although it’s not yet clear if a new flu variant is causing more severe disease than usual in people.

The Centers for Disease Control and Prevention said in its most recent update that an influenza variant, called H3N2 subclade K, is dominating. Of the 163 influenza A(H3N2) viruses, which are the most frequently reported strains, collected since September, around 89 percent belonged to subclade K, the agency said.
“We’re still in the middle of trying to figure out whether it’s producing worse illness or whether what we’re seeing is a large number of cases that are increasing, and then there’s a correspondingly similar increase in terms of the severe illness,” Andrew Pekosz, a professor and vice chair of the department of molecular microbiology and immunology at the Johns Hopkins Bloomberg School of Public Health, said in a news conference on Dec. 16.

In response to a question about which geographic areas in the United States are affected, Pekosz said, “Right now, we’re seeing clade K everywhere we are seeing influenza.”

There have been reports that the “disease is particularly strong in children” and reports of elderly having higher hospitalization rates, noting that researchers are trying to determine if the “clade K influenza virus is evading immunity” and making people more likely to get sick or if the variant is causing more severe illness on a “case-by-case basis,” Pekosz said in the news conference.

It’s not “completely clear” as to what the variant is doing so far, he continued. But what is clear to researchers is that flu “cases are increasing and we are seeing a lot of influenza cases” across the United States, Pekosz said.

His comments come as the World Health Organization (WHO) on Dec. 17 said that the clade K variant is also “driving infections” across the European continent, with it making up to 90 percent of confirmed influenza cases. Officials said there is no evidence that it is causing more severe infections so far.

“The flu comes around every winter, but this year is a little different,” said Dr Hans Kluge, WHO Regional Director for Europe, in a statement. “It shows how even a small genetic variation in the flu virus can place enormous pressure on our health systems because people do not have built-up immunity against it.”

The latest CDC data show that as of early December, flu activity was increasing in most parts of the United States. Some indicators have shown that it’s low overall, although the season has just started, according to the agency.

A number of diseases tend to peak in the winter. The list includes not only colds, flu, and COVID-19, but also norovirus—a highly infectious cause of vomiting and diarrhea. Norovirus cases have generally been trending up in the last month.

COVID-19 activity right now is very low, while RSV, or respiratory syncytial virus, is also low, according to the CDC.

“RSV activity is increasing in the Southeastern, Southern, and Mid-Atlantic areas of the country with emergency department visits and hospitalizations increasing among children 0–4 years old,” the agency said on Dec. 12.

The Associated Press contributed to this report.