Where the flu stands in the US

According to CDC, for the week ending Nov. 22, positive flu tests are up 5% nationwide and 3,264 patients were admitted to hospitals with influenza, which is an increase from the week prior. In addition, 2.5% of visits to a healthcare provider were for respiratory illnesses, which is also an increase from the week prior but still below the national baseline.

CDC estimates there have been at least 1.1 million illnesses, 11,000 hospitalizations, and 450 deaths from the flu so far this season. So far, no influenza-associated pediatric deaths have been reported to CDC for the 2025-2026 season.

During the week ending Nov. 22, there were 358 influenza virus cases reported by public health laboratories, 343 of which were influenza A and 15 of which were influenza B.

While CDC reports that overall flu activity is low in the United States, some areas of the country are seeing higher activity than others. For example, Louisiana and Colorado are seeing "high" activity of influenza-like illnesses.

Why experts are worried about a severe flu season

Generally, the first clue as to how the flu will spread in the United States is how the Southern Hemisphere fared with the virus during its winter season. This year, Australia had the worst flu season in its history.

"They had some pretty decent flu activity in many parts of the Southern Hemisphere," said Richard Webby, who studies the flu at St. Jude's Children's Research Hospital. "It actually lasted for a longer period of time — the tail of the season went on for longer than typical."

So far this flu season, the dominant strain of the flu in the Northern Hemisphere has been an H3N2 virus, CDC said. For the week ending in Nov. 22, CDC said that 82.3% of influenza viruses reported by public health labs were H3N2.

According to Webby, H3N2 viruses "tend to be a little bit more problematic. When we have an H3N2 season, we tend to have a little bit more activity, a little bit more disease at the severe end of the spectrum."

Currently, a new variant of H3N2, called H3N2 subclade K, is the dominant flu strain within the United States. The last time an H3N2 virus dominated the flu season in the United States was 2016-2017.

"There is basically a new variant of influenza circulating that has mutated a little bit," said Caitlin Rivers, an epidemiologist at Johns Hopkins. "And that means that it's just different enough from what your body or the vaccine may recognize that it can kind of get around those protections."

"There are very clear signals that it's going to sweep the world, and it's going to sweep the world quickly," said Derek Smith, a flu expert and director of the Center for Pathogen Evolution at Cambridge University.

Experts urge vaccination as soon as possible

Rivers noted that this year's flu shot "may not be as good a match as if we hadn't seen this new variant." However, she added that early data from the United Kingdom still indicates that vaccines make a difference.

In a preprint study, researchers at the U.K. Health Security Agency found this year's vaccine was 70% to 75% effective at preventing hospitalization from the flu in children and teens and 30% to 40% effective in adults.

"It's not like we're expecting to get complete loss of protection for the vaccine, but perhaps we might expect a little bit of a drop-off if this is the virus that sort of dominates the season, and early indications are that's probably going to be the case," Webby said.

"It is absolutely clear that the vaccine may not stop people getting infected with the new variant," Smith said, but it "will still help to protect against severe disease."

Rivers said she urges anyone to get the vaccine "as soon as possible. Because activity is low right now. But it is increasing. And it will continue to increase straight through until we hit peak, which is usually around the holiday season. There's no sense in waiting when we are barreling into flu season."

"We're likely a few weeks out from things getting really serious, and that means now is the time to get vaccinated, because it takes a couple of weeks for full protection," said Sam Scarpino, a public health expert at Northeastern University.

According to Scott Hensley, a microbiologist and influenza vaccine scientist at the University of Pennsylvania, "If there's ever a year to get a flu vaccine, this is the year."