How Bad Will Flu Season Be This Year?NEWS | 27 November 2025U.S. flu rates remain low, but experts are keeping an eye on a new strain that’s been linked to unexpectedly early and severe seasons in several other countries
Influenza is picking up in the U.S., and public health experts are bracing for signs of another brutal season with the virus.
Last year one of the worst waves of flu in recent decades took the U.S. by surprise. Japan and the U.K. are currently grappling with an unexpectedly early uptick in severe flu cases. A mutated strain known to cause severe disease may be behind some of the unusual activity. These warning signals, combined with interruptions in U.S. surveillance data and declining vaccination rates, have scientists worried.
“Influenza is no joke,” says Shaun Truelove, an infectious-disease epidemiologist at Johns Hopkins University. “Right now it’s time for everybody to get vaccinated.”
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The government shutdown interrupted national flu data reporting last month, leaving scientists in the dark. A spokesperson for the Centers for Disease Control and Prevention told Scientific American that data were collected during the shutdown, but publication of weekly flu reports and respiratory dashboard updates were paused between September 20 and November 8.
That’s a crucial period for tracking flu trends, Truelove says. Officials and scientists ramp up flu monitoring in October because “knowing what’s going on and how quickly rates are increasing will help us understand the impact on health care systems.” CDC’s spokesperson said that the agency will release data from that time gap to provide a complete picture of the season moving forward.
Amanda Montañez; Source: Centers for Disease Control and Prevention (data)
Flu season in the Northern Hemisphere typically begins around October and peaks between December and February. The CDC’s latest weekly flu report shows low but rising flu activity across the country. Some U.S. regions, including Puerto Rico and Louisiana, are seeing slightly higher rates of flu compared with other parts of the country, though this generally follows past fall patterns, says Helen Chu, a University of Washington physician and co-lead of the Seattle Flu Study. “In some years, we see earlier activity in the southeast and then it increases from there.”
An August CDC assessment of the upcoming U.S. respiratory season anticipated that flu would be moderate for all ages. Truelove and his colleagues at the Infectious Disease Modeling and Analytics Center also forecast several different flu scenarios for 2025-2026, which project lower peaks than last season.
Seeing back-to-back severe flu seasons is rare. And last year was a particularly bad flu season: at least 560,000 people were hospitalized, and 280 children died from the virus. A high-severity flu season might create higher levels of population immunity, Chu explains. “The spread of flu the following season should be less, but I don’t know that we can say that confidently because the flu virus mutates,” she says. “We may have something different this year.”
Scientists expect flu viruses to change slightly year to year, a process called antigenic drift. But some years, an unexpected mutation can make the virus different enough to cause more widespread disease. If the change is stark enough—a process called antigenic shift—it can trigger a pandemic.
A Concerning New Strain
Influenza that primarily infects humans come in two subtypes: A and B. Last year researchers saw two influenza A strains, H1N1 and H3N2, detected at relatively similar levels; influenza B viruses came onto the scene slightly later in the season. Wastewater monitoring this fall has detected both A and B subtypes. Influenza A H3N2 is currently the most frequently reported flu strain in the U.S.—and viral testing shows a particularly concerning variant makes up the majority of cases.
“Of the small number of strains that have been genetically analyzed, over half of the H3N2 strains belong to subclade K, the new variant first reported in Europe this summer and now causing influenza surges in many countries,” says physician Robert Hopkins, medical director of the National Foundation for Infectious Diseases.
Other countries show some warning signs. Australia reported a record flu season, with nearly 11 percent more cases than the previous year. Japan, meanwhile, declared an early “flu epidemic” in October. Influenza cases are also increasing in several European countries, and U.K. health officials announced that flu struck a month earlier than usual.
Preliminary data link the new K strain to some of this activity. Early data from Japan and the U.K. show that 90 percent of flu samples contained the mutated H3N2 virus. H3N2 tends to mutate more quickly than H1N1 and influenza B viruses, Chu says. Data from past seasons show that H3N2 is also associated with higher rates of hospitalization and death compared with the other flu viruses. It’s too early to determine if the K variant will result in more severe disease in the U.S., Hopkins says, but he adds, “It has certainly raised my level of concern.”
Health officials are also keeping watch for possible signs of international transmission in major U.S. ports, particularly as holiday travel picks up, Hopkins says. The reports from abroad are notable, he says, but it doesn’t necessarily mean the U.S. will follow the same pattern.
Vaccination Still Reduces Severe Disease
The current vaccines include multiple influenza A and B strains, but not the new K form because manufacturers and scientists determined formulas before it emerged. That means the shots may provide less protection than if the vaccine strains matched more closely, Hopkins explains.
“While this may affect the degree of reduction in risk of severe influenza you will receive from the vaccine, any risk reduction from the vaccine is greater than zero risk reduction you receive from not being vaccinated,” he says.
A preliminary U.K. report suggests that this year’s vaccines are 70 percent effective against emergency department visits and hospitalizations for young children. But Chu cautions that “this reflects kids who were infected right after getting the vaccine when their antibody titers [immunity] are highest.” Flu treatments, such as Oseltamivir (brand name Tamiflu) and Baloxavir, are still effective against the circulating strains, Chu says.
Flu vaccination rates have been trending downwards in recent years, and Chu and other clinicians worry the decline will continue this year. The CDC and other public health experts encourage everyone ages six months and older to get their flu vaccinations (“with rare exception for people who have contraindications,” in the CDC’s wording).
“We can’t always predict when the flu season will peak or how severe it'll be, but we can predict one thing with pretty good certainty,” Hopkins says. “People who are vaccinated are going to do better.”Author: Tanya Lewis. Lauren J. Young. Source
