INDIANA– Health officials across the United States, including Indiana, are on alert as a new, mutated strain of the influenza A (H3N2) virus, known as subclade K, has been detected in the state and is quickly becoming the dominant circulating strain.
Experts are concerned that recent mutations in the virus may reduce the effectiveness of this season’s flu vaccine against infection, though it is still expected to prevent severe illness.
The presence of subclade K in Indiana comes as global flu activity rises, with Japan having already declared a flu epidemic and the United Kingdom reporting unusually high case numbers.
The Challenge of Subclade K
The new version of the H3N2 virus, designated as subclade K, carries seven genetic mutations in its hemagglutinin (HA) gene—the protein targeted by the vaccine. These changes allow the virus to be considered “antigenically drifted” from the H3N2 strain included in the 2025–2026 seasonal flu vaccines.
- Vaccine Mismatch: The World Health Organization (WHO) determines the vaccine composition months in advance. Subclade K reportedly emerged at the end of the Southern Hemisphere’s flu season, after the strains for the Northern Hemisphere’s vaccine had already been selected, making a perfect match impossible.
- Widespread Circulation: According to the Global Initiative on Sharing All Influenza Data (GISAID), subclade K has already been submitted for sequencing by more than 30 U.S. states, including Indiana, demonstrating its rapid spread. Recent data from the Centers for Disease Control and Prevention (CDC) indicates that 86.8% of genetically characterized H3N2 viruses collected since late September belong to this subclade.
Protection Against Severe Illness Remains
While the mismatch may increase the risk of getting a milder infection, experts and public health data still strongly support vaccination.
- Reduced Severity: Early data from the United Kingdom shows that the flu vaccine effectiveness against flu-related hospitalizations has remained within expected ranges—estimated at 70% to 75% for children and 30% to 40% for adults. This is a crucial defense, as seasons dominated by H3N2 strains tend to be more severe, especially for the elderly and young children.
- Current Toll: Despite the early season timing, the CDC estimates that there have already been at least 1.9 million flu illnesses, 19,000 hospitalizations, and 730 deaths from flu across the U.S. this season.
How to Protect Yourself and Others
Flu season in the U.S. typically peaks between December and May. Cold, dry air and increased indoor gatherings during the winter holidays contribute significantly to the virus’s spread.
| Action | Why It’s Important |
| Get the Flu Shot | Still the best tool for preventing severe illness, hospitalization, and death, even with a partial mismatch. |
| Practice Good Hygiene | Wash hands frequently and cover coughs and sneezes to limit droplet spread. |
| Stay Home When Sick | The flu causes common symptoms—fever, chills, severe headache, body aches, and fatigue. Isolate yourself to protect the community. |
| Seek Early Treatment | If you test positive for the flu, antiviral medications like Tamiflu can lessen the severity and duration of the illness, but they must be started within 48 hours of symptom onset. |
