Flu season is arriving early and with a potentially dominant strain, prompting health officials across Europe and the United States to closely monitor rising cases and prepare healthcare systems for increased strain. An unusually early surge in influenza, alongside ongoing circulation of SARS-CoV-2 and RSV, is raising concerns about a challenging winter season. Experts are particularly focused on a newly prevalent A(H3N2) subtype, known as K, and its potential impact on vaccine effectiveness.
Respiratory viruses, particularly influenza, SARS-CoV-2, and respiratory syncytial virus (RSV), are placing significant strain on healthcare systems across Europe and the United States each winter. Understanding the current trends and potential impacts of these viruses is crucial for public health preparedness.
Influenza typically causes up to 50 million symptomatic cases and between 15,000 and 70,000 deaths annually on the European continent alone. While all age groups are affected, children tend to have higher infection rates and are often the first to contract and spread the virus within households, contributing to wider transmission. Approximately 20% of the population contracts the flu each year.
The influenza viruses that commonly infect humans fall into two main types: influenza A, which is more variable and responsible for most seasonal epidemics and all known pandemics, and influenza B, which primarily circulates among humans. Influenza A is further categorized into subtypes based on its hemagglutinin (H) and neuraminidase (N) proteins. Currently circulating subtypes include H1N1, including the 2009 pandemic strain which is now seasonal, and H3N2. These subtypes can evolve each season due to minor mutations, known as antigenic drift. Influenza B has two lineages: B/Victoria and B/Yamagata, with the latter rarely detected since 2020.
Because of this constant evolution, flu vaccines are updated annually to include a combination of three or four of the viruses expected to be most prevalent in the upcoming season.
Early Start to Flu Season This Fall
Table of Contents
This year, the flu season appears to be starting several weeks earlier than in recent years. For example, Japan has reported an unusually high number of cases since October.
The European Union is also experiencing an unusually early start to the flu season. Spain has already surpassed the epidemic threshold, weeks ahead of schedule.
A notable trend this season is the predominance of influenza A(H3N2) virus, specifically a subtype known as K. While influenza A(H1N1) was dominant in the Southern Hemisphere this year, and subtype K of A(H3N2) initially emerged later, it ultimately became the most prevalent strain in both the United Kingdom and Japan, accounting for approximately 90% of flu samples in those countries. Similar trends are now being observed in the United States and Canada.
This A(H3N2) K group (formerly designated J.2.4.1) has been detected on all continents and represents about one-third of all A(H3N2) viruses analyzed globally between May and November 2025, and nearly half within the European Union. This widespread presence highlights the potential for broader transmission.
A(H3N2) K: Evolving, But Not Necessarily More Severe
The genetic changes within the A(H3N2) K subtype are not the result of viral recombination or mixing. It’s essentially the same A(H3N2) virus that has circulated in the human population since 1968, but with some mutations in its genes. Influenza viruses evolve and mutate each season to evade our immunity, and the success of this process varies from year to year. The findings suggest that these changes do not appear to affect the effectiveness of antiviral medications, nor are they linked to increased virulence or disease severity.
In fact, Asian countries that are now reporting a decline in A(H3N2) K epidemics have not experienced unusually high illness severity. Analyses also indicate that the K subtype strains circulating in these countries are similar to those now present in the European Union. This is reassuring, as it suggests A(H3N2) K does not appear to be more virulent or cause more severe illness.
What Does This Mean for Case Numbers?
A(H3N2) has not been the dominant influenza virus in recent seasons – A(H1N1) has been more common. This could lead to lower population immunity, as people may not have been recently exposed to A(H3N2). Seasons dominated by A(H3N2) tend to be more severe, with reduced vaccine effectiveness and more serious cases in older adults compared to seasons dominated by A(H1N1).
How Well Will the Vaccine Protect?
Flu vaccines are developed each year using data from the previous year to predict which viral strains will be most dominant in the upcoming season. This year’s vaccine was designed to protect against two subtypes of A(H1N1), influenza B, and a class of A(H3N2) that predates the new K subtype.
Some analyses show a divergence between the new K subtype and the A(H3N2) strain included in the vaccine. However, real-world vaccine effectiveness data is currently limited. If vaccine effectiveness is reduced, more flu cases could be expected. Close monitoring this season is critical to determine the level of protection the current immunizations are providing.
Even if a mismatched A(H3N2) virus becomes dominant this winter, the vaccine is still expected to provide protection against severe illness, remaining a fundamental public health tool. The current risk to the general population is considered moderate, but may be higher for those more likely to develop severe illness – individuals over 65, those with underlying health conditions, pregnant people, and those with weakened immune systems.
While the new A(H3N2) K subtype doesn’t appear more virulent, the early start to the flu season and the prevalence of A(H3N2) suggest a potentially more challenging season, with increased hospitalizations and strain on healthcare services. This isn’t due to increased virulence of A(H3N2) K, but rather the potential for a higher number of cases.
Recommendation: Get Vaccinated Promptly
Even in seasons with lower vaccine effectiveness, vaccination still offers some protection and remains the most important way to reduce the risk of severe illness.
Therefore, vaccination is recommended for all individuals for whom it is indicated. Given that the epidemic has started early this year, and it takes several weeks after vaccination for full immunity to develop, it is advisable to get vaccinated without delay. Finally, practicing good hand hygiene and wearing a mask when you suspect you are infected can help prevent the spread to vulnerable individuals.
Ignacio López-Goñi, Professor of Microbiology. Member of the Spanish Society of Microbiology (SEM), Universidad de Navarra
This article was originally published in The Conversation. Read the original.
