ECDC Threat Assessment Brief: Assessing the risk of influenza for the EU/EEA in the context of increasing circulation of A(H3N2) subclade K

 

#18,954

Ten days ago, in Increasing Concerns Over A `Drifted' H3N2 Virus This Flu Season, we looked at early reports of a rapidly growing subclade (K) of seasonal  H3N2, which has sparked early and robust flu outbreaks in Asia, and which appears to be poorly reactive to this year's flu vaccine. 

A week ago, we looked an a UK HSA  Preprint: Early Influenza Virus Characterisation and Vaccine Effectiveness in England in Autumn 2025, A Period Dominated by Influenza A(H3N2) Subclade K, which was formally published today in Eurosurveillance.

While that preliminary study found `. . . reassuring early evidence that a programme using NH-strain enhanced vaccines offers protection against clinical influenza disease . . .', this is based on very limited data, and reliable VE (Vaccine Effectiveness) numbers won't be available for months. 

Today the ECDC published their own risk assessment, which reports that a vigorous and early flu season has already started, and `vaccination should proceed without delay.'

First the press release, followed by excerpts from the Risk Assessment. 

Threat Assessment Brief: Assessing the risk of influenza for the EU/EEA in the context of increasing circulation of A(H3N2) subclade K
Risk assessment
20 Nov 2025

Circulating respiratory viruses, including influenza viruses, SARS-CoV-2 and RSV, all contribute to pressure on healthcare systems during winter in the EU/EEA. In a typical season, influenza causes substantial morbidity in the European population, with up to 50 million symptomatic cases and 15 000 to 70 000 deaths annually.

All age groups are affected, although children have higher rates of illness and are usually the first to become sick and transmit the disease in their households, which can drive transmission in the community. It is estimated that up to 20% of the population contract influenza annually. This results in absence from school and work and a significant impact on healthcare systems. A higher impact is seen in closed settings such as long-term care facilities (LTCFs), where outbreaks of seasonal influenza can have high morbidity and mortality.

ECDC decided to assess the risk of influenza for the EU/EEA in the context of early circulation of seasonal influenza in the region and the recently emerged influenza A(H3N2) subclade K that is circulating globally. This is to raise awareness of potential implications and provide recommendations to public health authorities. However, considerable uncertainty remains around the likely public health impact of this subclade on the influenza season.

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Threat Assessment Brief - Assessing the risk of increasing circulation of A(H3N2) subclade K.pdf - EN - [PDF-533.55 KB]

While there is considerable uncertainty in this risk assessment, for now the risk to the general population is considered moderate. Those over 65 - or with comorbidities - are deemed as having greater risk, and reduced community immunity could result in increased pressure on health care providers.

I've posted some excerpts below, but you'll want to follow the link to read the full 10-page report.   I'll have a brief postscript after the break.

Risk assessment

Based on currently available information, ECDC assesses the risk for the general EU/EEA population from an influenza season dominated by A(H3N2) subclade K as moderate. 

ECDC assesses the risk as high for populations at higher risk for severe disease (people over 65 years of age,people with underlying metabolic, pulmonary, cardiovascular, neuromuscular and other chronic diseases,pregnant people or persons who are immunocompromised, and people living in closed settings such as LTCF).

Even if the individual risk of severe illness remains similar to previous years, a larger epidemic driven by lower immunity to infection could result in a higher absolute number of hospitalisations and increased pressure on healthcare services. This assessment may change as more data become available.

Recommendations

• Those eligible for vaccination, especially those at higher risk of severe disease, should get vaccinated without delay.

• Treating affected individuals with influenza antivirals early is essential to reduce the likelihood of complications and disease progression in populations at higher risk of severe disease. Antivirals become even more important in the context of a circulating influenza strain that may be poorly matched to the vaccine.Testing should guide antiviral treatment where possible, but strong clinical suspicion and the local epidemiology should also guide decisions to avoid delays that may reduce effectiveness

• Antiviral prophylaxis should be considered in outbreaks detected in closed settings, such as LTCFs, regardless of vaccination status.

• Hospitals and LTCFs should review their preparedness plans and enhance their infection prevention and control practices to mitigate against pressure to the healthcare system during the influenza season. Staff and visitors should use face masks within hospitals and LTCFs in periods of increased respiratory virus circulation.

• Countries should provide tailored communication to people on how transmission can be reduced and the impact of severe disease. This should be done through clear messages on vaccination, hand hygiene and respiratory etiquette.

• Countries should continue to report epidemiological and virological surveillance findings promptly via EpiPulseto support rapid assessment and response across the EU/EEA

Disease severity and impact

It remains uncertain whether influenza A(H3N2) will dominate throughout the 2025/26 season or whether co-circulation with A(H1N1)pdm09 and/or B/Victoria will occur. The EU/EEA has not experienced dominant circulation of A(H3N2) since the first half of the 2022/23 influenza season (Figure 2). The 2021/22 season was also dominated by A(H3N2) but with low overall levels of activity, following 2020/21 in which circulation of influenza was interrupted due to the COVID-19 pandemic [6].

Reduced recent exposure may lower population-level protection against infection with A(H3N2), particularly among young children who may have had little or no prior exposure to this subtype. However, protection against severe disease is likely to remain more robust due to cross-reactive immunity from previous influenza infections and vaccination, consistent with observations from past seasons and post-pandemic periods.

Even if the individual risk of severe illness remains similar to previous years, a larger epidemic driven by lower immunity to infection could still result in a higher absolute number of hospitalisations and increased pressure on healthcare services. Serological data for the 2025/26 season are not yet available, and these assessments therefore remain uncertain.

        (Continue . . . )

Fortunately, even a poorly matched vaccine is expected to provide some degree of protection - at least against severe infection - and so it is still very much worth getting.  

I got mine a month ago, but the flu shot is just part of my `flu prevention' routine each year.  I'll also continue to wear a face mask in public, and will liberally apply hand sanitizer at every opportunity. 

Admittedly, I'd do that every year. But with a drifted H3N2 virus in the mix - which often hits my age bracket the hardest - I'll be cinching my mask just a little bit tighter in the months ahead.