Detection of Influenza A(H3N2) Subclade K Confirmed: 2025 Vaccine Remains Effective Against Severe Disease

Original article: No es un virus nuevo: detectan subclado K de influenza A(H3N2) y la vacuna 2025 sigue protegiendo

In an official announcement, the Ministry of Health, through the Public Health Institute (ISP), reported the identification of the K subclade of influenza A(H3N2) in samples analyzed within the country. This discovery, occurring amidst a sustained decline in influenza activity in Chile, does not indicate a new threat but rather an expected evolution of the seasonal virus, against which the 2025 vaccine maintains its effectiveness in preventing severe cases and hospitalizations.

The detection resulted from a specific enhancement of genomic surveillance. According to details provided by the ISP, «at the request of the Ministry of Health and as part of the enhancement of genomic surveillance, the ISP intensified the sequencing analysis of influenza A (H3N2) samples in recent weeks. This effort confirmed the presence of the K subclade, a genetic variant of the H3N2 virus that has circulated widely in other regions of the world since August 2025.»

Furthermore, the agency anticipated that «it is likely that additional positive samples for this subclade will be identified in the coming days as sequencing analyses are completed.»

Epidemiological Situation in Chile

The announcement regarding the detection of this variant is not linked to a resurgence of the disease. On the contrary, health authorities emphasize that viral circulation continues to decline.

The report indicates that «the most relevant observation is that influenza activity in Chile has been steadily decreasing for several weeks, based on findings from the ISP’s respiratory virus surveillance. Syndromic surveillance indicators show that both cases of severe acute respiratory infection (SARI) and influenza-like illness (ILI) are also on the decline.»

Regarding the clinical impact, it is noted that «so far, there has been no increase in the clinical severity of cases or in hospitalizations related to influenza A(H3N2), consistent with international reports regarding the K subclade.»

What is Subclade K?

Health authorities aim to reassure the population by clearly explaining the nature of this viral finding.

«Subclade K is a variant of the influenza A(H3N2) virus. Influenza is a virus that changes naturally over time, leading to different ‘branches’ of the same virus, known as subclades,» clarifies the document issued by the ISP.

They also stress that «subclade K is not a new or different virus but rather reflects the expected evolutionary process of the seasonal influenza virus that is already circulating globally.

The key message lies in the protection offered by current vaccination. «Available evidence indicates that the 2025 influenza vaccine continues to provide protection against severe illness and hospitalization, and it is expected that this protection will be even greater with the 2026 vaccine,» states the press release.

Active Surveillance and Ongoing Recommendations

The Ministry of Health highlighted that «Chile’s influenza surveillance system will continue to closely monitor the evolution of the virus and its variants,» adding that «the detection of subclade K demonstrates that our genomic surveillance systems are functioning effectively to identify changes in circulating viruses.»

In light of this situation, recommendations for the public remain unchanged and focus on vaccination, especially for high-risk groups: adults aged 65 and older, pregnant women, individuals with chronic illnesses, children aged between 6 months and 5 years, as well as healthcare workers.

«Vaccination remains the best tool for preventing severe illness, hospitalization, and death from influenza. For high-risk populations, antiviral treatment is advised,» the statement reminds.

Additionally, the importance of basic preventive measures is reiterated: frequent handwashing, respiratory etiquette when coughing or sneezing, staying home if experiencing respiratory symptoms, and promptly consulting in cases of serious symptoms.

The Ministry of Health committed to keeping the public informed about any significant changes in the epidemiological situation of influenza through its official channels, emphasizing the routine and expected nature of such findings within the epidemiological surveillance of a constantly evolving virus like influenza.