Sudden fever, a severe cough and acute respiratory distress are the familiar onset symptoms of seasonal flu, which has affected WHO’s European Region with unusual intensity and speed this year. As the Northern Hemisphere enters winter, health systems are struggling to manage an epidemic driven by an aggressive, genetically mutated influenza strain A(H3N2) subclade K.

But a new report by the European Centre for Disease Prevention and Control (ECDC), published on Friday, concluded that current seasonal influenza vaccines remain effective against this new strain of the virus. Health experts urge vulnerable groups to get vaccinated.

The current influenza outbreak is dominated almost entirely by the A(H3N2) subclade K, which now accounts for up to 90 per cent of all confirmed cases in the region, according to data from WHO’s European Regional Office (EURO). Subclade K marks a “notable evolution in influenza A(H3N2) viruses,” having undergone genetic drift and displaying several amino acid changes in the hemagglutinin protein (the “key” the virus uses to unlock and enter human cells). Due to these slight changes, antibodies acquired from previous infections or older vaccines may not recognise the virus as effectively.

“Flu comes around every winter, but this year is a little different,” stated Hans Kluge, WHO Regional Director for Europe, in a press release on Wednesday. Although there is no evidence that it causes more severe disease, Kluge explained that the small genetic variation in the virus places “enormous pressure on our health systems because people don’t have built-up immunity against it.”

Flu season gains early momentum

With the new strain spreading quickly, the current influenza season began approximately four weeks earlier than in previous years. High or very high activity is now being reported in at least 27 of the 38 countries being monitored by the WHO European Region. These countries range from EU Member States such as Ireland and Slovenia to Kyrgyzstan and Montenegro.

“This is expected to cause a significant burden in terms of morbidity and mortality, as observed in past years, and target groups should be vaccinated rapidly,” said Bruno Ciancio, an ECDC senior expert, in response to a query from Health Policy Watch.

WHO data shows that the strain has been detected in more than 34 countries globally over the last six months. While it is most prevalent in the European and Western Pacific regions, its expansion has also been confirmed in the WHO South-East Asia region. Since October, A(H3N2) subclade K sequences have been reported in Nepal, India and Thailand, as well as in the WHO African and Eastern Mediterranean Regions.

Vaccines remain primary shield against severe illness

With cases expected to continue rising, likely peaking in late December or early January, protecting the most vulnerable is paramount. Public health authorities are emphasising urgently that vaccination is the most important protective measure for vulnerable groups, including adults aged 65 and over, pregnant women, people with chronic conditions and healthcare workers.

Although recent reports suggest that subclade K shows “reduced reactivity” to current vaccines, it remains effective in preventing severe health outcomes, the ECDC concluded. “The current influenza vaccine is not perfectly matched to circulating strains, including H3N2. However, the primary aim of vaccination is to prevent severe disease, and effectiveness against severe outcomes is expected to be preserved,” explained ECDC expert Ciancio.

According to the preliminary data published by the ECDC for this flu season, vaccine effectiveness in preventing influenza cases that require medical attention at the primary care level ranges from 52% in children (ages 0–17) and 57% in adults (ages 18–64). For the critical group of individuals aged 65 and older, it was not possible to estimate vaccine effectiveness separately, due to the low number of influenza cases so far considered in the study. ECDC experts emphasized, moreover, that the findings rely on small sample sizes from nine participating countries and have low statistical precision (e.g. wide confidence intervals). Efficacy could fluctuate as the season progresses.

According to a ECDC analysis released only this week, vaccines significantly cut hospital admission rates in the last 2024-25 flu season. The modeling study found that seasonal vaccines were 70-75% effective at preventing hospital admissions among children aged 2-17 years and 30-40% effective in adults. Using computer simulations, the analysis estimated that vaccination programmes prevented 26-41% of flu-related hospitalisations among adults aged 65 and over across European Union (EU) countries between August 2024 and June 2025.

Low vaccination rates remain public health concern

Within the EU, only Denmark, Portugal, and Ireland met the 75 percent target for older adults during the reporting period. The ECDC projects that a 75% vaccination rate can prevent up to three-quarters of flu-related hospitalisations, significantly reducing the strain on public health systems.

Influenza vaccination rates for high-risk groups in other EU countries remained below WHO targets in the past seasons, as ECDC data from the most recent available season (2021-2022) shows. The overall median influenza vaccine coverage for adults aged 65 years and older was only 43 percent.

The scale of the challenge is huge. Recent interim data from Germany’s Robert Koch Institute (RKI) for the 2024-25 season found that flu vaccination coverage among German adults aged 60+ declined from 39.7-34.5% since 2020-21, the first year of the COVID pandemic, reaching its lowest level in over 17 years. And at the same time, a severe flu season can strain hospital staff and capacity in already overburdened health systems.

Combating the flu requires a multi-layered approach, experts underline. Slowing transmission requires proactive public health and social measures alongside vaccination, says WHO. These steps, proven effective during the COVID-19 pandemic, include staying home if unwell, wearing a mask in public if symptomatic, maintaining regular hand hygiene, and improving indoor ventilation.

European health systems have “decades of experience managing influenza,” said Kluge, striking a note of optimism alongside the WHO warning. “We have safe vaccines that are updated annually, and we have a clear playbook of protective measures that work.”

Image Credits: European Union, WHO/Europe , World Health Organization, European Union.