The effectiveness of the flu vaccine against the K variant

The 2025-2026 flu season has burst into force in Europe, moving between three and four weeks ahead of the previous two years.

Despite initial concern about the emergence of a new variant of the virus, the influenza A(H3N2) subclade Kpreliminary data from the European Center for Disease Prevention and Control (ECDC) is reassuring: Current vaccines are working to prevent infections.

According to the VEBIS multicenter study, the doses available in the European Union offer protection against the predominant strain, A(H3N2), which ranges between 52% and 57%. In general terms, the effectiveness against any type of flu is in the 44%.

It should be remembered that these percentages refer to the protection offered by the dose against infections that require medical attention in primary care.

Now, the vaccine not only serves to prevent mild or moderate cases, but mainly so that, if the patient becomes infected, have less risk of being hospitalized –This study does not assess this aspect.

In this sense, the report highlights that the circulation of the virus is especially high among children between 5 and 14 years old, while hospitalizations are mainly affecting those adults over 65 years.

By age groups, the effectiveness against the A(H3N2) virus is estimated at 52% for children under 17 years of age and a 57% for adults between 18 and 64 years old.

Now, it should be noted that, at the moment, it has not been possible to calculate a specific figure for those over 65 years of age. due to the low number of cases included in this initial stage of the study.

Spain, key piece in the European study

The data that support these conclusions come from a surveillance network that includes primary care centers in various countries, including Spain (both nationally and specifically in Navarra)along with France, Germany, Ireland, Italy, Portugal, the Netherlands and Romania.

The study is based on a methodology of “negative test”, where patients with respiratory symptoms who test positive for flu are compared to those who test negative, analyzing your previous vaccination status.

Caution and vaccination

Despite the good results against the A(H3N2) variant, the ECDC warns that the efficacy against other subtypes, such as A(H1N1)is currently imprecise (it stands at 16%) due to the small size of the sample analyzed until week 49 of 2025.

Experts emphasize that these estimates are preliminaries and could vary throughout the season depending on the evolution of the virus and the possible decrease in immunity over time.

However, the message is clear: these results support the need to vaccinate key populations as soon as possible to mitigate an impact that is expected to be considerable this winter.

In addition to the vaccine, the ECDC recommends the use of antivirals and masks in healthcare environments and care centers to stop the transmission.