It is the same H3N2 virus that has been circulating among humans since 1968, but with some mutations in its genes. This is predictable, as the flu virus evolves each season through mutations to evade our immune system. Influenza A H3N2 is one of the variants that circulates every year. These subtypes can vary each season due to small mutations or antigenic drift. The variability of the flu virus explains why vaccines are prepared each year with a mix of three or four viruses that circulated the previous year. In the European Union, the flu appeared unusually early. In Spain, for example, the epidemic threshold was already exceeded a few weeks earlier than expected. Another novelty is the clear predominance of the A H3N2 virus, specifically the K subclade. This subclade, previously known as J.2.4.1, has been detected on all continents. It represented a third of all A H3N2 viruses analyzed worldwide between May and November 2025, and almost half in the European Union. Subclade K is different but not necessarily more harmful. The genetic changes in subclade K of the A H3N2 virus do not come from recombination or mixing of viruses and do not represent a significant alteration. The Secretariat of Health reported that, to date, no confirmed cases of influenza A H3N2 subclade K have been identified in Mexico. In a statement, it noted that the National Epidemiological Surveillance System (SINAVE) maintains permanent monitoring and analysis for the timely detection of any potential case. In some years, the vaccine is more successful than others. Mutations in this subclade do not seem to affect the efficacy of antivirals and are not associated with greater virulence or severity of the disease. The K subclade or variant, which is currently circulating in Europe and the United States, was not identified in Mexico. The population was urged to go to health centers and medical units and vaccination posts to get the corresponding winter season vaccines, particularly for COVID-19, influenza, and pneumococcus—according to their corresponding scheme. These vaccines help reduce risks, prevent complications, and protect health, the department stated. These doses are primarily aimed at girls and boys, older adults, pregnant women, health personnel, and people with risk comorbidities, the Secretariat of Health noted. The new variant has been circulating since 1968 with some variants. Every winter, respiratory viruses—especially the flu, SARS-CoV-2, and respiratory syncytial virus (RSV)—affect all age groups, although children have the highest infection rates and are usually the first to get sick and spread it in their homes. It is estimated that up to 20 percent of the population contracts the flu each year. The flu viruses that typically infect humans belong mainly to two types: influenza A—the most variable, responsible for most seasonal epidemics and all known pandemics—and influenza B, which circulates almost exclusively in humans. There are many subtypes of influenza A, depending on how their hemagglutinin (H) and neuraminidase (N) proteins combine. The subtypes currently circulating in humans are H1N1—including the 2009 pandemic variant, now seasonal—and H3N2.
