The 2023 southern hemisphere influenza vaccine

Each year we get a new vaccine for each hemisphere. The design gets settled six months out, and sometimes a lot can change in that time. So how does the 2023 situation look?

The recipe for 2023

The World Health Organization (WHO) recommended in September that this season’s vaccine’s “active ingredients” come from the influenza virus strains listed below. Each contains the protein that sticks out of the surface of the influenza virus (hemagglutinin or HA). This list represents the four main influenza viruses that are likely to circulate during ‘flu season’.

There is a bit of a caveat to this. One of the four, a lineage influenza B viruses that can all be grouped under the name of the “Yamagata lineage”, kind of disappeared as a result of lockdowns and travel shutdowns. The global influenza conveyor belt, on which these viruses are wholly reliant, stopped enough, and for enough time that this lineage of viruses may have become extinct. None have been seen for years. But for now, and until we’re sure there are none ticking over in poorly sampled parts of the world, the best flu vaccine still contains four ingredients (quadrivalent).

Will the flu vaccine work well this year?

That’s always something to hope for, but we’ll only really know much later, often after the end of the influenza season, because of the work required to find that out.

Take a guess!!

Okay! Geeze. So grumpy.

From what we can see right now, though, it looks like being immunised against Influenza B and the A/H3N2 components of the vaccine will provide good protection for what’s circulating, in Australia at least.

The image below shows that the past two A/H3N2 vaccines for the southern hemisphere (SH) have contained the same H3N2 strains (pink and green arrows marking their position on a phylogenetic tree that compares lots of HA genes from 2014 to this week, from all over the world. This tree image is a screenshot from the excellent NEXSTRAIN public online resource.

The vaccine strain sits in an evolutionary spot that should generate immunity after vaccination, which can reduce the chance of serve disease if we’re infected by related strains. WHO-affiliated laboratories have done some early work, and their findings support the gene sequencing-related picture below.

Of course, any flu season could be mixed up if some major change occurs to the virus strain. That can’t be perfectly predicted, though. Change is something these viruses are always doing, and the child viruses that emerge with the best ability to escape from our immunity are the evolutionary winners that go on to spread.

We’ve all seen a lot of this story with another virus, SARS-CoV-2, over the past 3 years, where vaccines have largely not kept pace with virus evolutionary change. With influenza, though, it’s worth remembering that the world has seen human generations’ worth of exposures, and so every one of us has a baseline of immunity to seasonal influenza viruses.

Images generated at the Nextstrain H3N2 website and saved for pasting here. The bold ‘X‘ indicate past and current vaccine strain viruses.

Similarly, the Influenza B lineage virus (just showing the Victoria lineage known below) vaccine components haven’t changed (indicated by green and pink arrows) this year, and according to the WHO report, the vaccine is likely to afford good protection once you’re immunised.

Images generated at the Nextstrain FluB\Victoria lineage website and saved for pasting here. The bold ‘X‘ indicates past and current vaccine strain viruses.

With the Influenza A/H1N1 component, a vaccine change was made in 2023 (green arrow) because these viruses had drifted away from what was around in 2022 (pink arrow). Our immune responses to last year’s virus wouldn’t have been as protective if we got infected this year.

You can see below that the two arrows are quite separated – which indicates a difference in those HA proteins. That change was also confirmed by lab work presented in the WHO Report. Real-world studies using human sera from people immunised last year, more recently tested against these more evolved A/H1N1 viruses, found a change was needed. And so it was made.

Images generated at the Nextstrain FluB\Victoria lineage website and saved for pasting here. The bold ‘X‘ indicates past and current vaccine strain viruses.

So no one will get sick from influenza in 2023?

I wish it were that simple, but no. Of course, the virus doesn’t stop evolving once we’ve settled on our vaccines, a few of us don’t mount a good immune response to the vaccine, and many of us simply don’t get flu shots. Also, the most recent influenza viruses, on the right-hand side of those phylogenetic trees, have continued to move right; they have kept evolving. And that’s not just to evolve away from eh immune response of immunised people, but also from all those infected. So it’s a constant problem that newer vaccines will fix in the near future.

For now, no vaccine is not going to help, while getting vaccinated is safe and will definitely prepare your immune system. I got my quad vaccine this week, so I’m prepared. Because almost no one around me wears masks and there is no useful filtration on most of the air I share with other human groups (both methods reduce viral load and thus harms), vaccination remains one of the main ways I can add a layer of protection to prevent me from getting seriously ill, which is my goal.

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