This is a graph I didn’t know I wanted to make. But now I have, I needed to make it! It shows the influenza virus detections reported to laboratories in Australia over nearly a quarter of a century. This is mostly about peaks and some ripples in the pond of flu circulating, caused by the stone that was SARS-CoV-2 (the virus that causes COVID-19).

The graph above highlights four interesting points that I would like to expand on.

The pandemic and the new gunslinger in town

When SARS-CoV-2 emerged from China, it didn’t just cause fear, drama, sickness, and death. It also stirred up the balance among all the other well-established endemic human respiratory virus groups.

The many distinct human rhinoviruses, adenoviruses, enteroviruses, the handful of RSVs, MPVs, influenza viruses, parainfluenza viruses, the other established coronaviruses and lots of other viruses and bacteria that also interact with them via direct and indirect actions, all got jostled about while their usually willing human hosts dutifully queued for their next season of each, suddenly developed a bad case of the immune system response, otherwise occupied by a new host mate – SARS-CoV-2. That has taken some time to settle down. We’ve seen multiple years’ of earlier or larger or squashed seasons of a whole range of viruses and bacteria. Maybe it will 100% be back to normal this, or maybe not. We’ll know the answer afterwards.

An example of the scale of SARS-CoV-2’s interference is illustrated in ❶ of the graph above. No influenza peaks. There were also dramatically weakened peak seasons of other viruses. For flu, the seasons didn’t happen in 2020 or 2021. There were still scant positives being detected around, thanks to international travel that trickled on even during our border lockdowns. Households, frontline workers, and daycare for children helped keep other endemic viruses at slightly higher numbers. And so during that time, whole cohorts of newborn children were born who had never been infected and built up any immunity to that list above, but especially to the influenza virus.

A rising baseline: more lab testing or changed virus circulation?

Swing right to ❷; this shows an area where the summer trough, our traditional interseasonal period, didn’t return to zero. This isn’t new, as we saw it over the summer of 2019, and you could argue that it has been on an upward trend since the 2016 flu season. But it’s really clear now.

So, something(s) is different; the question to ask is, what? Two thoughts for now.

Have the viruses or their hosts undergone significant changes in a few years?

Remember that there are three distinct flu viruses that co-circulate each season: A/H1N1, A/H3N2, and B/Victoria. Virus changes that affect transmission during the summer would likely need to impact all at once, which is getting a bit unlikely.

Is it an increase in summer travel from the northern hemisphere or to the north? Or could it be some other change of behaviour(s) that’s causing more infections during warmer months when we traditionally think of as a ‘flu season’ in Australia (sorry tropical regions of Australia and the world, I do realise that your season lingers on, and your variants keep emerging during your wet season; as usual for colonials, let’s just gloss over your existence for now).

I don’t think these sorts of changes are the driver. There are no signs of a dramatic shift in summer travellers or changes in behaviours that would expose us to more flu in the summer.

I think that after we implemented more lab-based pathology testing following the #flunami of 2017, testing became easier to find and even faster, which created a bit of a feedback loop. Especially in the sphere of rapid point-of-care molecular diagnostics. Phew!

Doctors know they can achieve a good result quickly, which in turn helps them prescribe antivirals for flu and SARS-CoV-2 more efficiently. Faster drug prescription means a better likelihood the drugs will be effective. Doctors will use that test. They’ll order it because they know it’ll be helpful in managing their patients. Tests that take days or a week for an acute infectious disease are less valuable because they have to make a call one way or the other before the drug effectiveness window closes.

I’m suggesting that, apart from extensive testing in our big, beautiful, plentiful, well-equipped pathology labs, Australia may also be seeing the impact of changed pathology test ordering behaviour in the flu data, perhaps trading off some pathology test use stewardship. Who can say? However, this might be what drives more lab tests over the traditional trough of the flu season than we once did. Just a theory.

Traditionally in Australia, flu peaks just before July

If we examine ❸ in the figure above, we can see the blue vertical line, which is aligned with July, sits just ahead of the flu peak for 2003. Following these blue lines, you can see they sit in the same spot every year. Sometimes, they’re a bit close, sometimes a little further, but the flu peak generally follows the start of July.

These are monthly figures, so we won’t see the fine detail of exactly when after July. But I don’t think we really need to for this example.

While the start of the exponential epidemic curve may begin earlier or later, or the peak may be lower or higher, or its base may be fatter or thinner, the pattern of peaking is fairly predictable.

Until we got a new cowboy in town. SARS-CoV-2 and the COVID-19 pandemic.

Flu in post-pandemic town

After the pandemic floodgates opened in New South Wales, Australia’s first post-pandemic flu season took off again, and it was a doozy. No surprise – travel is essential for flu viruses to keep mutatin’ and mixin’ and getting the boosts they need to mix up a new cocktail of antigenic novelty.

This travel-fuelled variant milieu is the equivalent of SARSV-CoV-2’s forward-moving evolution we see in which each new variant replaces the previous one over time as one becomes too ‘well known’ by our immune system and is outshone by the next new young thing with trendy outerwear.

From ❹ in the graph, we can see that after the pandemic rolled in 2022, flu season peaked just before July in the 2022 and 2023 winter seasons. There isn’t a huge difference—at least not when using chunky monthly data and this type of graph—but it was consistent over those two years. Then it moved back to just after July.

We haven’t seen a peak for the 2025 winter flu season yet – and it’s clambering more than spriting its way upwards right now. But the season is starting to take off. However, I think we’ll probably peak after July again this year. If so, we’re back to normal flu peak times. All we need to sort out is these weird “bigger than ever” early start summer flu numbers and larger areas under the curve. For my money, those aren’t a big mystery, though – more testing reveals more iceberg.

But, I stand to be corrected by my betters as ever.😉

So what does this mean?

Well, it gives you a water cooler topic – ‘flu peaks just after July in Australia’. It is clear the pattern of flu virus detection has been gradually changing for a decade. But so has our capacity to test and perhaps our medical willingness to prescribe a test. When there’s a helpful drug that needs to be administered early and requires speed and volume, that’s a good thing.

I think it’s really interesting how much one virus can mess us up physically, emotionally, mentally, and politically. And how it can mess so thoroughly with its delicately balanced viral peers, all coexisting in a steady state of battle with the collective immune systems of newborns, young and old alike.

So much more goes on than sick people during a pandemic.

As we’ve moved out of the novelty phase of this latest pandemic, we’re left with a virus among us doing all the things we know it does and being surprised each time it comes back. But we’re not when flu season rolls around each year.

We haven’t put into practice some things we’ve learned. And we’ve slid back down the energy slope to the state of lowest potential. All those great ideas seem like a distant memory. This is all to say that a lot of this 👆 probably happened during the 1918 influenza pandemic, and we’re just as prepared for the next pandemic now as we were for the ones that followed. So maybe we’re not doing too badly! As long as you don’t expect us to improve as we go.

Reference

1. National Notifiable Disease Surveillance System
   https://nindss.health.gov.au/pbi-dashboard/
2. First combination COVID-19 and influenza self-tests approved for Australia
   https://www.tga.gov.au/news/media-releases/first-combination-covid-19-and-influenza-self-tests-approved-australia
3. Is it COVID or the flu? A new test that just hit pharmacy shelves will give you the answer
   https://www.sbs.com.au/news/article/is-it-covid-or-the-flu-a-new-test-that-just-hit-pharmacy-shelves-will-give-you-the-answer/pmh4bx4rk