As of November 21, 2025, seasonal influenza activity remains low in the United States but is increasing. A new influenza A(H3N2) virus subclade J.2.4.1, also recently renamed “H3N2 subclade K,” was identified by CDC in August 2025. Influenza viruses in this subclade have small changes (mutations) that make their hemagglutinin gene different and have been antigenically characterized as “antigenically drifted” in comparison to other recently circulating A(H3N2) viruses, including the virus selected as the A(H3N2) component of the U.S. 2025-26 seasonal influenza vaccines.

During seasons when circulating influenza viruses have drifted from influenza vaccine viruses, individual and population-level immunity to the drifted viruses might be low and annual influenza vaccine effectiveness might be reduced. The effect of antigenic drift on annual influenza vaccine protection is not easy to predict; even in past years with drifted circulating influenza viruses, substantial protection against the drifted influenza viruses has been observed. Influenza vaccination continues to be recommended for the 2025-26 influenza season. Additionally, at this time, there is no indication that A(H3N2) subclade K influenza viruses are resistant to available influenza antiviral medications. Healthcare providers should continue to initiate prompt treatment with influenza antiviral medications for patients with confirmed or suspected influenza who are hospitalized; who have severe, complicated or progressive disease; or who are at higher risk for influenza-associated complications.

CDC tracks and analyzes global influenza activity year-round. Multiple influenza viruses usually co-circulate each season, so influenza vaccines are formulated to protect against three different seasonal influenza virus types/subtypes, an A(H3N2) virus, an A(H1N1) virus, and an influenza B virus. In late 2025, reports noted a rapidly emerging group of A(H3N2) viruses called “H3N2 subclade K.” These viruses have genetic differences (10 additional mutations in the hemagglutinin gene) compared with the J.2 subclade A(H3N2) virus selected in early 2025 to produce this season’s annual U.S. influenza vaccines. These differences might result in reduced vaccine effectiveness against these A(H3N2) subclade K viruses.

Based on U.S. influenza surveillance data, which include illness and laboratory surveillance, national seasonal influenza activity is increasing. So far during the 2025-26 influenza season, the majority of influenza A(H3N2) viruses detected have been subclade K. From September 28, 2025, through the week ending November 22, 2025, U.S. public health laboratories tested 9,682 specimens; of 2,371 that tested positive for influenza viruses, 95% were influenza A and 5% were influenza B. Among 3,086 influenza A virus specimens subtyped, 77% were influenza A(H3N2) and 23% were influenza A(H1N1)pdm09. Among 91 influenza A(H3N2) viruses collected since September 28, 2025, that underwent additional genetic characterization at CDC, 87% belonged to subclade K. In the past, influenza A(H3N2) predominant influenza seasons have been associated with more influenza-associated hospitalizations and deaths in people 65 years and older and in young children compared to other age groups. It is too early in the season to predict which influenza viruses will be most common and in what proportion, or how well the annual influenza vaccines will work throughout the 2025–26 influenza season in the United States.

CDC is conducting studies to determine whether antibodies elicited by this season’s influenza vaccine in humans will protect against these A(H3N2) subclade K viruses. Early estimates of 2025-26 annual influenza vaccine effectiveness in the United Kingdom against influenza-associated hospitalization remained within expected ranges of 70-75% for children and 30-40% for adults, suggesting that influenza vaccination remains an effective tool in preventing influenza-related hospitalizations this season.

During seasons when circulating influenza viruses are drifted from viruses represented in the annual influenza vaccines, influenza vaccination continues to provide benefits, including: 1) protection against severe influenza illness, hospitalization, and death; 2) protection against other circulating influenza viruses represented in the influenza vaccines; and 3) help to reduce the overall community spread of influenza. Influenza vaccine effectiveness networks are collecting real-world data to produce early estimates of vaccine effectiveness in the United States once influenza activity increases.

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