Happy FluMas, ya filthy animals! With a new flu variant – subclade K – dominant, Australia and New Zealand are seeing a very unusual bout of seasonal influenza (flu) as we head towards the end of 2025. If you’ve been following me on my socials, you’ll already know we’ve seen a record number of flu detections this year, but instead of a long season that still ends, it’s flaring up again. What’s that about?

🎶It’s beginning to feel a lot like 2019 and 2024🎶

If you recall, Australia had a huge year in #Flunami2019. It was the biggest on record. Then 2024 beat it. 2025? “Hold my beer!”

2019 and 2024 had two things in common – a higher summer case load in their preceding years (2018 and 2023) than usual, which preceded – foretold? – a very big winter of flu detections. Not necessarily a more clinically severe year, but lots of infected people, all unwell to some extent

Humans of all shapes, sizes, genders, colours and beliefs, I give you 2025.

NOTE
Are these truly increased case numbers or just better detection of normal summer flu levels by laboratories with better post-pandemic-surge testing capacity and doctors now used to sending a sample off to see if it’s a SARS-CoV-2 (the cause of COVID-19; treatable if you start early), RSV or a flu (treatable if you start early) infection causing that illness?
(That was a very long sentence)
I don’t have a definitive answer.

Flu2025 in summary

Let’s step through what’s been happening in December.

I wrote earlier in the year about our Flunami in July. You can see that in the big peak of our seasonal influenza curve – the dashed thick red line below. But rather than fall away, cases remained high in August and September.

Instead of the annual epidemic returning to baseline levels, cases stopped declining in October and have started to rise again in November, December, and perhaps also in January (still a few days left). Just to note, there is always some flu around, just usually at very low levels outside the epidemic peak period.

In fact, Australia recorded the second-highest number of flu detections* for an August or September month on record**.

The previous graph doesn’t show it, and this one isn’t perfect either, but 2018 also saw a rise in December compared to November, which was in turn higher than baseline.

Of all years from 2001, only four show noticeable summer rises. 2022 and 2024 (below) offered a glimpse of this rise, but only after both first fell to levels much closer to a normal baseline.

And 2018 (below). Which had a similar pattern to 2025, although, if you look at the y-axis, 2018 recorded fewer than a quarter of the detections. The (very small) epidemic peak stopped falling in spring and then started rising in summer. This preceded a very big 2019 winter season that began around Easter, earlier than usual. At play here was a small 2018 season after a huge 2017 season, giving Australians quite a while for past flu’munity™️to fade, essentially setting us up for a big next season.

Those asterisks above (* and **)

*Most flu is detected using nucleic acid amplification tests (NAATs). You probably know the commonest of these – the polymerase chain reaction or PCR. These don’t detect infectious virus; they detect its genetic material. We know that the influenza virus’s genetic material isn’t always with us, and it doesn’t persist without some replication, so PCR (RT-PCR, to be pedantic) is a good surrogate for the presence of an infectious virus and thus for an infection. For the record, PCR is super-specific (it doesn’t detect viruses it isn’t designed to detect), it’s fast, and its semnsitive – but not so sensitive that it can detect a virus you just breathed in, but which didn’t start replicating in your airways (even with the second ‘A’ in NAAT, we still need enough virus to begin with).

**Australia’s national infectious diseases data, here taken from the flu component of the publicly available National Notifiable Diseases Surveillance System (NNDSS), starts in 2001. Before that, Australia still detected influenza, but the non-NAAT methods were very slow (virus culture-based), had lower throughput, were less sensitive, and so gave us a much smaller representation of all the cases in hospitals and the community. Even after 2001, the numbers aren’t anything like an exact indication of the numbers infected – but the trends tell us if cases are rising or falling or dragging out, and they are what to watch.

Australian flu numbers, year-on-year

First up: the annual totals of all flu detections each year. As we can see, 2025 takes the tissue box.

PCR testing has, without a doubt, allowed us to better capture more cases. After the world stopped travelling during the peak of the pandemic (2020/2021) – travel is essential for influenza to spread, evolve, and emerge anew – numbers appear to have risen year-on-year. But be aware that during the pandemic, pathology laboratories also gorged on additional – necessary – testing capacity to detect SARS-CoV-2, flu and RSV, making it faster and easier for your doctor to get results and for you to get treatment, whether in hospital or via your GP. So that will also contribute to the bigger annual tallies.

You can see in 2018 that after a massive 2017,m we had a quiet year. Perhaps teh viruses didn’t evolve in time for a new season, and so our previous levels of immunity kept a lid on things. But 2019 was huge, then was dwarfed by 2024 and now 2025. 2025 is a bit different, though, as we’ll discuss later.

For the record, the 3 different viruses are:

➡️FluA subtype, H1N1
➡️FluA subtype, H3N2
➡️Flub B lineage, Victoria

These are each further named according to where and when they were detected & characterised – each is from a single person.

For example: A/Fujian/411/2002(H3N2)

Here I’m defining a ‘variant’ as one of those viruses, eg H3N2, that ‘looks’ different enough to our immune memory to escape it & infect us..compared to another variant of the same H3N2 virus which we were previously infected by and developed immune memory to.

Think of ‘virus’ as the bag holding your marbles. Each marble is a teeny bit different from the other – some are a lot different – they are the variants, that make up ‘the virus’.

Next, the same numbers, plotted on a long axis and broken down by month from 2001 to 26DEC2025. I’ve also included the dominant influenza viruses from that year. See that pattern? Each seasonal flu year is dominated by a different virus. A mixture of immunity waning, but I think this is more key, virus novelty. New variants of an A(H1N1), an A(H3N2), or a fluB emerge from evolutionary pressures. These pressures are from our immune systems, and their need to replicate.

If a new variant can dodge our past immunity while still being a functional virus that transmits well, bingo, bango, it will “escape” because it’s a poor match for those old antibodies and immune cells. Same as we’ve seen with SARS-CoV-2.

Wherever there is such a novel respiratory virus, there will be new epidemics. And if the novelty is strong enough, the epidemic will occur outside of our usual seasonal patterns. Novelty is a key driver of flu patterns.

Flu cases by jurisdiction

If we break those numbers down by jurisdiction (still monthly), we can see how Australia doesn’t experience flu season the same way everywhere.

For example, in December, Queensland and the Australian Capital Territory have seen cases continue rising above November’s levels, while Western Australia has an epidemic peak – albeit a fat one – that rises and falls in a way you might expect. The hotter, more tropical Northern Territory has maintained a near-constant level of flu without a clear epidemic peak.

If we break down the totals by jurisdiction, there’s nothing much more to see. The rate of cases per 100,000 people in each region is also fairly similar, although a bit lower in Western Australia – perhaps they test less, or there’s just something special about the population density or about being way over there!

What’s driving the summer surge, and is it related to an earlier flu season in parts of the northern hemisphere?

By most accounts, it’s an A(H3N2) variant belonging to a group of related variants called subclade 2a.3a.1 (K), and ‘yes’. A(H1N1) is also present up north, but A(H3N2) is making up about 70–90% of flu detections.[6,7]

‘Subclade K’ variant viruses appeared late in the Australian 2025 flu season, after the A(H1N1)-dominated peak was declining. The fact that they have driven a rare unseasonal summer surge speaks to how different they must be from past A(H3N2) variants we’ve had.

Virus sequences show the earlier emergence of this new variant in the United States, the world’s flu travelator. In Europe and the United States, subclade K dominates the A(H3N2) variants (90% of the top 5 variants).[6,7]

How does the current vaccine hold up against subclade K infections?

It’s hard to tell how effective a vaccine is early in the season, and early results can change as the season progresses.

As you might expect, a novel variant that wasn’t around when the vaccine components were selected was a bit of a curveball. And despite this, an evolved, or ‘drifted’, flu variant that escapes some vaccine-induced immunity, early estimates suggest that the vaccine still provides some protection [9], especially against severe illness requiring hospitalisation.[8]

The big question – is subclade K a ‘killer flu’?

Well, yes, the pointy end (see below) of the infection outcomes triangle is death, confirmed as due to a flu infection. But is the current novel variant surging globally without regard for season, more likely to cause death than an average seasonal flu virus infection? No, it doesn’t seem like it.[4,8]

At least not with the data to hand. Flu can cause death normally, and severe disease and systemic neurological and cardiac effects in a ‘normal’ year. But watch this space.

Cases are predominating in those aged 5 to 14 years according to the European Centre for Disease Prevention and Control (ECDC).

Flu-related hospitalisations are occurring at a rising rate in all age groups, but especially in those aged over 65 years (ECDC).

The World Health Organization (WHO) noted…

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

Disease Outbreak News, WHO
Seasonal influenza – Global situation
10 December 2025

The flu baseline

A couple of random thoughts.

What if that “higher-than-expected” baseline we’ve been seeing (graph below)- yes, even before the COVID-19 pandemic – is the real one and that the previous lower levels of testing or use of less sensitive methods just gave us a false impression of flu cases always dropping back to very low numbers after the seasonal epidemic?

One thing against that idea is the spring-summer of 2022, which almost reached those low levels again.

Alternatively, we may have just had a lot of big years as our population regains immunity following lockdowns, which shut flu down during the ‘Infection Pause‘.

Or what if 2017’s extensive and severe flumageddon changed something about our immunity, imprinting us in some way that has been causing problems ever since?

As I said – random thoughts!

References

1. National Notifiable Disease Surveillance System (NNDSS)
   https://nindss.health.gov.au/pbi-dashboard/
2. Australian Respiratory Surveillance Reports (ARSR)
   https://www.cdc.gov.au/resources/collections/australian-respiratory-surveillance-reports
3. European Centre for Disease Prevention and Control (ECDC)
   https://www.ecdc.europa.eu/en/seasonal-influenza
4. Extended influenza seasons in Australia and New Zealand in 2025 due to the emergence of influenza A(H3N2) subclade K viruses
   https://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2025.30.49.2500894
5. Mortality, morbidity, and hospitalisations due to influenza lower respiratory tract infections, 2017: an analysis for the Global Burden of Disease Study 2017
   https://www.thelancet.com/journals/lanres/article/PIIS2213-2600(18)30496-X/fulltext
6. Weekly US Influenza Surveillance Report: Key Updates for Week 50, ending December 13, 2025
   https://www.cdc.gov/fluview/surveillance/2025-week-50.html
7. Communicable disease threats report, 13-19 December 2025, week 51
   https://www.ecdc.europa.eu/en/publications-data/communicable-disease-threats-report-13-19-december-2025-week-51
8. Seasonal influenza – Global situation, 10DEC2025
   https://www.who.int/emergencies/disease-outbreak-news/item/2025-DON586
9. Early estimates of seasonal influenza vaccine effectiveness against influenza requiring medical attention at primary care level in Europe, week 41 – 49, 2025
   https://www.ecdc.europa.eu/en/news-events/early-estimates-seasonal-influenza-vaccine-effectiveness-against-influenza-requiring