If you feel like everyone around you has the flu, it’s not your imagination.
As of January 9, the Centers for Disease Control and Prevention (CDC) estimates that at least 15 million people have contracted the flu this season. Of these, 180,000 people have been hospitalized and 7,400 people have died from the flu, including 17 children.
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Phoenix Children’s Hospital has seen a spike in influenza cases since mid-December 2025, said Dr. Monica Abdelnoor, a pediatric infectious disease specialist.
“The CDC, [currently] “Although we estimate this year’s influenza season to be of moderate severity, the incidence rates we are seeing are similar to, and in some cases slightly higher than, those observed during the severe 2024-2025 season. It remains possible that this influenza season will ultimately be classified as severe,” she told Live Science via email.
Although the CDC reported a slight decrease in the number of laboratory-confirmed influenza cases in the first week of 2026 compared to the last week of 2025, Madad told Live Science that “it is too early to declare that we have passed the peak.” Influenza activity in the United States typically peaks between December and February. The CDC also noted that the slight decrease in cases may be related to an expected decrease in doctor visits and testing during the holidays, rather than an actual decrease in the spread of influenza.
What is subclade K?
Much of the talk about this year’s influenza season has centered on something called “subclade K.”
To understand what this is, it’s important to know that influenza viruses are constantly evolving, acquiring small genetic mutations as they spread. Madad said scientists are using a system similar to a family tree to track how the influenza virus changes.
The large branches of this tree are called “types”. This includes influenza A and B, a broad group of influenza viruses. These large branches split into smaller branches called subtypes, such as the A(H3N2) lineage and the B/Victoria lineage.
The letters H and N, used to distinguish subtypes of influenza A, represent two proteins on the surface of the virus, hemagglutinin (H) and neuraminidase (N). There are many possible versions of each protein, 18 and 11 versions respectively, resulting in a wide variety of influenza viruses, but only a few of which routinely infect humans.
Further down the branches, scientists identify “clades,” groups of viruses that originate from a common ancestor and share similar genetic characteristics. Subclades are even smaller groups within a clade (new “twigs” on the tree) that share recent mutations.
This influenza season, the CDC reported that most influenza cases are caused by influenza subtype A (H3N2). Of more than 430 samples of influenza A(H3N2) viruses collected since late September 2025, more than 91% were identified as part of a closely related group called subclade K.
Is subclade K a “superinfluenza”?
The answer is no.
Subclade K has different mutations in parts of the virus that our immune systems normally recognize, Maddad said. Because of this, our immunity to past influenza infection or vaccination is lower than it would be if the subclade were more “well-known.” This allows the virus to spread more easily, resulting in more cases and, in turn, more hospitalizations.
But that doesn’t automatically mean subclade K is more deadly or causes more severe disease than other circulating influenza viruses, Madad said. He noted that current epidemiological data indicate that subclade K causes less severe disease than other seasonal influenza strains currently circulating.
“This year’s flu symptoms are similar to those seen in previous seasons,” Abdelnoor added.
People with the flu commonly experience fever and chills, cough, stuffy nose, muscle and body aches, headache, and fatigue. In severe cases, influenza infection can lead to complications such as sepsis and pneumonia. Adults 65 and over. Young children, especially children under 2 years of age. A pregnant person. Additionally, people with chronic health conditions such as asthma, diabetes, and heart disease are most at risk for severe flu complications.
Is this year’s flu vaccine effective, or is it too late to get it?
Madad said the influenza vaccine “might not be as perfect for subclade K as we thought,” but stressed that vaccination still plays an important role in preventing severe disease. Abdelnoah said getting the flu vaccine every year reduces the risk of serious complications such as sepsis, pneumonia and serious neurological conditions such as seizures.
Data from the Southern Hemisphere, where flu season arrives before the Northern Hemisphere, suggests this year’s flu vaccine is as effective as past vaccines in reducing the risk of hospitalization from influenza, Abdelnour said. A CDC report released in September 2025 found that this past flu season, the introduction of the influenza vaccine in the Southern Hemisphere resulted in a 50% decrease in flu-related medical visits and hospitalizations.
“This is within typical ranges, and we would expect to see a similar level of protection in the United States,” Abdelnoor said.
Unfortunately, fewer people are getting the flu shot this year, especially children, Madad said. “Only about 42% of children in the U.S. have received a flu vaccine so far this season, down from about 53% during the same period in 2019-2020.” As of Jan. 3, about 44.1% of adults said they had received a flu shot.
Maddad said low vaccination rates could have real consequences. During the 2024-2025 influenza season, 280 children in the United States died from influenza-related illness, and about 9 in 10 of them were not vaccinated against the virus. Madado, who is also a mother of four children, said all of her children receive flu shots every year.
Although flu season started in the fall, Abdelnoor stressed that “it’s not too late to get a flu shot.” “Given the current surge in cases, we strongly urge you to get vaccinated now to protect yourself and those around you.”
Mada echoed that advice, urging people who haven’t yet gotten a flu shot this season to do so, especially adults over 65, those who are pregnant, those with weakened immune systems, and those with chronic health conditions that put them at higher risk of severe illness.
How else can you protect yourself from the flu?
Vaccination is just a layer of protection. Mada emphasized wearing well-fitting masks in crowded indoor spaces and improving ventilation during indoor gatherings. Additionally, you should stay home when you’re sick to avoid the risk of spreading the flu to others. These measures will help limit the spread of not only influenza, but also other respiratory viruses such as COVID-19 and RSV (respiratory syncytial virus).
Mr. Abdelnoor also mentioned the importance of practicing good hand hygiene. “Particular attention needs to be paid to childcare environments such as schools and daycare centers,” he said. “This includes encouraging children to get their flu shot, encouraging hand washing with soap and water for at least 20 seconds, and keeping children home when they are sick to prevent infecting others.”
Is there a cure for influenza?
For people who become sick, antiviral drugs such as Tamiflu can help. These prescription medications can reduce the severity of symptoms and shorten the duration of illness, especially when taken within the first day or two after symptoms begin. There is evidence that it may be particularly helpful for people hospitalized with influenza.
The CDC recommends prompt treatment for people who have suspected flu symptoms and are at high risk for serious flu complications, including pregnant women and people with chronic health conditions such as asthma, lung disease, diabetes, and heart disease.
Madado said anti-influenza drugs are “underutilized” and urged people at high risk of severe influenza, or those caring for someone at high risk, to talk to their doctor about antivirals as soon as they suspect they have been infected. The good news is that current data show that subclade K is still responsive to standard antiviral drugs doctors use for influenza, she said.
This article is for informational purposes only and does not provide medical advice.
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