Credit WHO Global Respiratory Rept #556
#18,982
For the past 5 weeks we've been following reports of unusual seasonal flu activity - both in the Southern and Northern Hemispheres - which has been attributed to the recent emergence of a `drifted' H3N2 virus (subclade K) over the past few months in the Southern Hemisphere.
While the WHO has already selected two new strains (H1N1 and H3N2) for next year's Southern Hemisphere flu vaccine, we are going into winter with a vaccine that was formulated before this new variant emerged.
Since decisions must be made more than 6 months in advance on what flu strains to include, there is always the chance of mutations emerging, particularly in the H3N2 subtype - which typically evolves at faster rate than H1N1 (see The Enigmatic, Problematic H3N2 Influenza Virus).
While there are reasons to believe the existing vaccine will be at least `partially protective' - at least against severe infection (see UKHSA Preprint: Early Influenza Virus Characterisation and Vaccine Effectiveness in England in Autumn 2025, A Period Dominated by Influenza A(H3N2) Subclade K) - we won't have useful VE (vaccine effectiveness) numbers until January.
Over the past month we've seen reports of unusually early, and robust, flu outbreaks in places like Japan, South Korea, China, and much of Europe. Some countries are now urging the public to wear face masks in public, and many school closures have been reported.
Somewhat remarkably, Australia's flu season - which would normally have ended in August - continues (albeit well off the highs of July) during their summer (see Australian CDC chart below)
The Australian Respiratory Surveillance Report (Dec 1st, 2025) describes this unusual extended flu season, and attributes it to the emergence of the drifted H3N2 virus.
Nationally, the number of influenza cases has been consistently increasing since late October, which is unusual for this time of year. Current case numbers remain considerably higher than observed at the same time period in previous seasons (Figure 6).
– Several factors may be contributing to this out-of-season increase but the main driver is most likely influenza A(H3N2), in particular clade 2a.3a.1, subclade K
– Subclade K has developed seven new mutations, making it different enough from the 2025 southern hemisphere influenza vaccine reference strain (subclade J.2). As a result, the influenza A(H3N2) subclade K strain now circulating is less responsive to antibodies produced by this year’s vaccine compared to the influenza A(H3N2) strain circulating earlier in the season.
– Despite this, the influenza vaccine still provides important protection against severe illness and complications from influenza. The current influenza A(H3N2) subclade K viruses also show reasonable match to the 2026 proposed southern hemisphere vaccine strain.
– This situation reflects what has recently been reported in northern hemisphere countries,where the influenza season has started unusually early.
Overnight the WHO published a lengthy DON (Disease Outbreak News) report on this `drifted' H3N2 virus, and the impacts it is having around the globe.
As we've noted often, surveillance and reporting isn't always as timely or robust as we'd like (see From Here to Impunity), and so the WHO is operating on limited data.
Due to its length, I've only posted some excerpts. Follow the link to read it in its entirety. I'll have a bit more when you return.
Seasonal influenza - Global situation
10 December 2025
Situation at a glance
Seasonal influenza (‘the flu’) is an acute respiratory infection caused by influenza viruses that circulate globally and year-round. It can cause illness ranging from mild to severe, sometimes resulting in hospitalization or death. Seasonal influenza activity has increased globally in recent months, with an increased proportion of seasonal influenza A(H3N2) viruses being detected. This rise coincides with the onset of winter in the northern hemisphere and an increase in acute respiratory infections caused by influenza and other respiratory viruses typically observed at this time of year.
Although global activity remains within expected seasonal ranges, early increases and higher activity than typical at this time of year have been observed in some regions. Seasonal influenza viruses, including A(H3N2) viruses, continually evolve over time.
Since August 2025, there has been a rapid increase of A(H3N2) J.2.4.1 alias K subclade viruses detected from several countries based on available genetic sequence data. These subclade K viruses have several changes from related A(H3N2) viruses. Current epidemiological data do not indicate an increase in disease severity, although this subclade marks a notable evolution in influenza A(H3N2) viruses. Early estimates suggest that the influenza vaccine continues to provide protection against hospital attendance in both children and adults, even though its effectiveness against clinical disease during the current season remains uncertain.
Vaccines remain essential, especially for people at high risk of influenza complications and their caregivers. Even if there are some genetic differences between the circulating influenza viruses and the strains included in the vaccines, the seasonal influenza vaccine may still provide protection against drifted viruses and the other virus strains included in the vaccine. Vaccination is still expected to protect against severe illness and remains one of the most effective public health measures. WHO continues to monitor global influenza activity and influenza viruses, supports countries in surveillance capacity and updates guidance as needed.
(SNIP)
Genetic characteristics of recent seasonal influenza viruses
Influenza A(H1N1)pdm09 and influenza B/Victoria lineage viruses continue to circulate in all regions albeit at low levels.
Influenza A(H3N2) viruses
Based on genetic sequence data available in GISAID, a mixture of A(H3N2) haemagglutinin (HA) clades and subclades are currently circulating globally; however, there has been a recent and rapid rise in a particular subclade of A(H3N2), J.2.4.1 (alias subclade K Nextclade/Nextstrain nomenclature). A(H3N2) subclade K viruses have genetically drifted from related J.2.4 viruses and have several amino acid changes in their HA in comparison. Detections of subclade K viruses are increasing in many parts of the world, with the exception, to date, of South America. Subclade K viruses were particularly evident from August 2025 in Australia and New Zealand and have now been detected in more than 34 countries over the last 6 months.
WHO risk assessment
Seasonal influenza activity has increased globally in recent months, and influenza A(H3N2) viruses are predominant. This rise coincides with the onset of winter in the northern hemisphere. Epidemics and outbreaks of seasonal influenza and other circulating respiratory viruses can place significant pressure on healthcare systems. Although global activity remains within expected seasonal ranges, early increases and higher activity than typical at this time of year have been observed in some regions.
Seasonal influenza could place significant pressure on healthcare systems even in non-temperate countries. Genetically drifted influenza A(H3N2) viruses, known as subclade K viruses, have been detected in many countries. While data on how well the vaccine works against clinical disease this season are still limited, vaccination is still expected to protect against severe illness and remains one of the most effective public health measures.
While likely hampered by reduced reporting over the Thanksgiving Holidays, last week the CDC's FluView reported that the vast majority of H3N2 viruses characterised in the United States since the end of September are now subclade K.
During Week 48, of the 348 influenza viruses reported by public health laboratories, 341 were influenza A and 7 were influenza B. Of the 280 influenza A viruses subtyped during Week 48, 44 (15.7%) were influenza A(H1N1)pdm09, and 236 (84.3%) were A(H3N2).
- A new influenza A(H3N2) virus subclade J.2.4.1, also recently renamed "H3N2 subclade K," was identified by CDC in August 2025. These viruses have small changes in their hemagglutinin gene and have been antigenically characterized as "antigenically drifted" in comparison to the virus selected as the A(H3N2) component of the U.S. 2025-26 seasonal influenza vaccines.
- Among 91 influenza A(H3N2) viruses collected since September 28, 2025, that underwent additional genetic characterization at CDC, 86.8% belonged to subclade K.
Give the historic severity of H3N2 - particularly in the elderly - and the rapid spread (and unusual persistence in Australia) of this emerging subclade, I'm glad I got this year's flu vaccine, even if its effectiveness ends up being diminished.
I am admittedly augmenting its protection by wearing a KN95 in public, and using copious amounts of hand sanitizer.
While influenza is always unpredictable, the early signs suggest we could be in for another severe season, so anything you can do to avoid infection should be a high priority.
