On Monday, November 17, 2025, we published a warning: this year’s flu season may be worse than the official line. We pay close attention to the southern hemisphere, which offers us an early hint about what could be coming. Due to those early signals from abroad and a lagging U.S. surveillance system, we alerted you with the title: “Flying Blind into Influenza Season.” Now, only days later, we must add a new chapter. A mutated version of the influenza A H3N2 virus–dubbed Subclade K flu– is here. Our hypothetical warning is now real!
What is Subclade K Flu and Why Should You Care?
There are two basic kinds of influenza virus:
- Type A influenza infects animals, birds and humans. There are surface proteins H and N that lead to categories such as H1N1 or H3N2. These viruses can mutate and make it hard to predict if a vaccine will work. People tend to get sicker from type A flu.
- Type B influenza primarily infects humans and does not mutate as easily at type A. Subtypes include Yamagata or Victoria.
What do we know about Subclade K flu?
- This influenza A H3N2 variant collected 7 new mutations over the summer, after this year’s vaccine had already been created. Experts call it a genetic “drift, not a shift.” That hopefully means no pandemic.
- The Subclade K flu was detected early in Japan. The country declared an epidemic weeks ago. Schools were closed. But the CDC was pretty much nonfunctional due to the government shutdown.
- By early November, the UK Health Service was tracking Subclade K flu. Now reports suggest that 87% of the UK samples are this variant.
- The influenza vaccines that were formulated many months ago do not fully protect against the Subclade K flu mutations.
- Why does this matter? It means that the new mutated flu strain that is starting to circulate in the US may be somewhat resistant to this year’s influenza vaccines.
- How well will a flu shot work? That is the BIG question no one can yet answer. The UK Health Security Agency suggests that the vaccines are “70 to 75% effective” in preventing hospitalization in kids and “30 to 40% effective in adults.”
Those numbers aren’t great, especially in adults. Remember, that’s hospitalization. How effective are this year’s vaccines at preventing influenza at all?
It will take months before we have an answer to that question. Even in a “good” year when the vaccines match the circulating viruses, effectiveness is not great. We have analyzed influenza effectiveness data for many years. Here is a link to our report going back to 2004.
What Could the Subclade K Flu Do To You?
We warned you about the possibility of a bad flu season on November 17, 2025. Why were we concerned? Because of what happened in Australia, New Zealand, Chile and South Africa. The southern hemisphere always tips us off to what is coming because they those countries are six months ahead of us. But, as far as we know, those countries were not fighting the Subclade K flu. It was just the “regular” H3N2 influenza strain. In theory, the vaccines were working…but people still suffered long and hard. Here is a link to that article.
If you are older, suffer from a chronic illness or are immunocompromised, a flu shot still makes sense. It may not provide great protection, but it appears that it could help somewhat.
Dr. William Schaffner at Vanderbilt University Medical Center is a renowned expert on influenza. He notes that:
“…even if there is not a close match, use of the vaccine continues to prevent hospitalizations, intensive care unit admissions and continues to help keep people out of the cemetery.”
What Else Can you Do This Winter?
If you start to feel sick with typical influenza symptoms (sore throat, fever, muscles aches, cough, fatigue), get tested! In our November 17 alert, we suggested having an OTC home flu test on hand, just in case you start to experience symptoms. Here are some options:
- iHealth COVID-19, Flu A&B 3-in1 Antigen Rapid Test
OSOM Covid Flu Test Combo – 3-in-1 Rapid Flu A B and Covid Home Test Kit
- CorDx TyFast 10 Mins Flu A/B & Covid 19 Multiplex Rapid Test
The People’s Pharmacy is reader supported. When you buy through links in this post, we may earn a small affiliate commission (at no cost to you).
- Early diagnosis of influenza is important because antiviral drugs only work well if they are started within 24 to 48 hours after symptoms begin.
- Your healthcare provider will want to know the results of your rapid flu test before prescribing an antiviral medicine. The new mutation should not impact antiviral effectiveness.
- If you test positive for influenza, even if it is Subclade K flu, your doctor may be able to prescribe oseltamivir (Tamiflu), zanamivir (Relenza) or baloxavir (Xofluza). These are not great, but they may reduce hospitalization and/or serious outcomes.
- If you develop influenza, please stay home! Do not spread the virus to friends or family, even if there are holiday celebrations. If you must go out, please wear a high efficiency N95 mask.
- Make sure you have high-efficiency home filters. And if you must be in a large public space, take along a carbon dioxide meter so you can check the air quality of the place you are in. Here is an article about CO2 meters. Remember, there are other viruses besides influenza!
- Listen to our interview with Dr. Roger Seheult. He will suggest some strategies for strengthening the immune system and what to do if you catch the flu.
Final Words About Subclade K Flu:
We hope that this year’s flu turns out to be mild. But we are entering the high-risk season for lots of upper respiratory tract infections. In addition to influenza viruses, there are lots of cold viruses, RSV (respiratory syncytial virus), Human Metapneumovirus (HMPV), whooping cough (pertussis), SARS-CoV-2 (COVID) and Mycoplasma pneumoniae (walking pneumonia).
Out ability to avoid one of these infections depends in part on our immune system, but even people who are extremely healthy and take very good care of themselves can catch one of these nasties. You may find our eGuide to Colds, Coughs & the Flu of some help in dealing with symptoms if you do catch something this winter. This online resource can be found under the Health eGuides tab.
If you have found this article of value, please share it with friends and family. Most people will not have heard about Subclade K flu. Many will assume that their flu shot will work really well. They may be surprised to discover that their symptoms are caused by a flu bug that has mutated. We hope this article will prepare you and your acquaintances for any contingency. Thank you for supporting our work.
