Why is flu season so bad this year?
By: Riley Duignan

You’ve probably experienced it first hand and seen it throughout all your friends, the flu season in the 2025-2026 season has been terrible. With medical professionals recording record high numbers for flu in children, as well as alleged highest number of influenza related doctors visits in the past 30 years. But why is this?

According to the CDC, from October 1, 2025 to January 31, 2026, the flu/influenza virus has caused around 22-38 million illnesses, 280,000-590,000 hospitalizations, and 12,000-60,000 deaths. Somewhat similar statistics to the prior 2023-2024 influenza statistics, but possibly reaching even higher in deaths and hospitalizations. In Colorado, this season has been record breaking. With reports being the highest number of influenza hospitalizations in the 20 years of reporting and documentation of this data. According to CU, in the week ending December 27, 2025., 831 people had been hospitalized from influenza, which is the highest number they’ve seen since data has begun to be documented. An ILI (influenza-like illness) map of the US shows that the highest ILI activity levels as of March 7th, 2026 is: Missouri, Colorado, Oregon, Idaho, New Mexico, Iowa, Ohio, Tennessee, Nebraska, and South Carolina. (CDC Weekly Influenza Report).

This influenza/flu season is so bad because of the influenza A(H3N2) strain. This year, it’s responsible for around 90% of the Influenza A cases this year. This Influenza A(H3N2) isn’t just its normal strain either, it’s mutated, with its mutated strain being known as subclade K. This mutation made it easier to evade already existing immunity within preexisting vaccines and bodily immune systems. It has its typical influenza symptoms like sore throat, fatigue, severe body aching, coughs, and fever, the difference is its higher contagion from its resistance to previous immunity. But for many it may appear as more severe and long lasting as a normal strain of Influenza A, and is more likely to spiral into pneumonia or Bronchitis than other influenza strains. In addition, CPR states that not only is this strong strain attacking, but vaccination rates are also decreasing. “Changes at higher government levels, the CDC and HHS, have led to conflicting information about vaccines,” he said. “I think that’s led to lower rates of flu vaccines, and that means more severe illness and more spread.” (May, Haylee, 31 Dec 2025).

Furthermore, the 2025-2026 year has seen erratic and intense seasonal changes, being another cause of such high rates of this new Influenza A strain. Warm winters leave people more vulnerable to the seasonal changes of the upcoming season, and causes earlier and more severe infections. For example, El Nino, causes warmer temperatures especially for west and midwest locations, causing delayed winters and erratic temperature/weather changes. This is seen through the late 2025 and 2026 time period, and is predicted to continue through summer. This could cause record high summer temperatures and severely erratic winter and spring weather. Because of this unusual winter, there was a surge in early and severe influenza cases this 2025-2026 period. For example in Colorado on March 5th, Colorado hit a high of 68 degrees during the day, the next day, the temperature dropped all the way down to 32 degrees and snowy. This rapid and extreme change in temperature and weather can cause other issues as well such as asthma flare ups and attacks, severe allergies, migraines, etc.

With this, there are also many ways to prevent such rapid spread of sickness. To help prevent yourself and others around you from catching this strain, it’s as easy as washing your hands extremely regularly, wearing masks if you are sick or near someone who is infected (especially in airports), and getting annual flu shots and vaccinations.
Stay safe and take care of yourself!

Sources
https://www.cdc.gov/fluview/surveillance/usmap.html
https://www.cdc.gov/flu/whats-new/flu-summary-addendum-2023-2024.html#:~:text=The%20CDC%20estimates%20that%20the%20following%20occurred,120%2C000%20influenza%2Drelated%20hospitalizations%20*%207%2C900%20influenza%2Drelated%20deaths
https://www.cdc.gov/mmwr/volumes/75/wr/mm7509a3.htm#:~:text=Introduction,risk%20for%20influenza%2Dassociated%20complications.
https://news.cuanschutz.edu/news-stories/colorado-ranks-among-the-highest-states-in-the-country-for-flu-an-emergency-room-physician-describes-why-the-2025-26-flu-season-is-hitting-hard#:~:text=Colorado%20is%20in%20the%20midst,remain%20higher%20than%20prior%20years.
https://www.who.int/emergencies/disease-outbreak-news/item/2025-DON586#:~:text=Based%20on%20genetic%20sequence%20data,to%20date%2C%20of%20South%20America.
https://pmc.ncbi.nlm.nih.gov/articles/PMC3770759/#:~:text=Dependence%20of%20the%20peak%20timing,temperature%20of%20the%20prior%20season.&text=We%20posit%20that%20warm%20winters,interesting%20avenues%20of%20further%20research.
https://e360.yale.edu/digest/study-finds-link-between-flu-risk-and-wild-weather-swings#:~:text=The%20international%20team%20of%20researchers,swings%20in%20weather%2C%20click%20here.

What Is Subclade K, the New Flu Strain Doctors Call “Severe”?

https://www.yalemedicine.org/news/subclade-k-what-to-know-about-this-years-flu