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Ok, I'll try to be clear and keep it simple.

Flu has a bunch of proteins, but here we're just going to look at the 2 that are on the surface of the virus:

• Hemagglutinin (H) : this one is used by the virus to attach to cells and enter them, that's how you get infected
• Neuraminidase (N). When the virus highjacks a cell to create more copy of itself, the new virus need to leave said cell. So imagine baby viruses hanging on a cell. Neuraminidase is used by the baby viruses (virions) to detach from the 'mother' cell.

So, you've probably heard about H5N1 or others. That's how we identify the strains, by which hemagglutinin and which neuraminidase they're using.

Now onto the species barrier. There are different types of flu (A, B and C). B is a classic human flu, C you barely need to talk about, basically just a common cold. Flu A is the one we get mostly sick from and is the one that tends to cross species barrier and cause pandemics.

For example, usually yearly vaccines contain a couple of A strains and a B strain, whatever was most prevalent in the other hemisphere 6 months ago.

Back to species barrier - flu is not only human, can be avian, swine...

Crossing the species barrier can happen one of two ways:

• either the virus adapts to our cells, mostly through the H protein. That's typically what happens when chicken get infected. They have the same receptors as birds on their cells, so they can get infected by avian flu. But chickens also have receptors similar to humans. So in chicken, the virus can adapt to human receptors and become infectious for humans. That's usually H5N1.
• either a host sensitive to several strains can get infected by several viruses. For example, a pig could get infected by avian flu, human flu and swine flu. Then, upon packing new viruses, the cell doesn't know which is which. Imagine having to repack unlabelled DVDs into the right anthology. So mixes can happen, and that's usually how we get scary viruses such as H1N1 in 2009 (although swine flu by itself is usually H1N1 to be fair)

I hope this makes things a bit clearer, there is much more to it, such as the fact that inter species adaptation also occurs via the polymerase because the replication temperature in different hosts is not the same etc.

If you have more questions or things are unclear feel free to ask :)

Edit to add:

Oh I forgot to say - Spanish flu was H1N1. It was a crazy pandemic mostly because such a mix that could infect humans had never been around before

And all H1 for example are not the same, which is confusing when talking about different H1 strains - it's a bit like saying H1 would be a dog and H5 is a cat - it doesn't tell you if the H1 you're looking at is a husky or a poodle

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Or, for something a little easier to digest. Spanish Flu was Swine Flu, or derived from it. Spanish Flu isn't really a thing. There is a reason why you hear it called Spanish Flu but I don't have time to go into it. As far as I know but I could be wrong, "Spanish Flu" would that have been spread around Europe would have been imported from the USA via troops afflicted with the H1N1 virus.

Yeah sorry I got carried away 😅

Yes, a mix that happened between pigs and chickens in Kansas is the main hypothesis 🙃

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(For anyone curious about above statement) Here’s a source why it’s called Spanish Flu. Spain was one of the only countries who admitted they had a problem since the rest of the world wanted to keep up appearances for the war. https://www.cdc.gov/flu/pandemic-resources/1918-commemoration/key-messages.htm

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Awesome info

Very informative. Thank you.

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thank you so much

ohhh

>For example, usually yearly vaccines contain a couple of A strains and a B strain, whatever was most prevalent in the other hemisphere 6 months ago.

This bit of vaccine logic has never made sense to me given the speed of flu virus mutation. I mean you can get over the flu in a couple of weeks but then soon after pick up a brand new mutated strain. But it takes 6 months to pump out a vaccine for one set of strains that odds are are obsolete by now and one's body is already immune to through natural exposure?

I mean I understand what you're saying that there are virus sets in the vaccine one wouldn't necessarily run into but still. 6 months seems to be an awfully long lag time compared to the flu's mutatuon rate and even human natural immune response and adaption rate. So yeah this particular bit of logic never made sense.

But please if you have a sensible counter argument for why someone with a functioning immune system should bother then I'm listening.

Why this is relevant is your above explaination seemed to take yearly flu innoculation for granted.

The idea behind using those strains is that 1. We can't really do better, 2. Even though these strains are going to mutate before those 6 months, it is unlikely that all of the virus' proteins will mutate so drastically that all the antibodies we've made against the vaccine will be useless. After all, it only takes one good antibody for your body to be able to fight.

That being said, I agree that you don't need the vaccine if you have a fully functioning immune system. Idk elsewhere but in my country the vaccine is only recommended for people with weak immune system (elderly, auto immune disease etc).

The only reason you'd want to vaccinate even though you're not part of these groups of people is to protect the ones you know. That's why health workers have to get the vaccine, and you may want to get it if you're in contact with immuno-weak people (grandparents, newborn...). Not to protect yourself but to protect them.

Edit to add: some years, people who pick which strains to put in the vaccine simply get it wrong, we get a completely different strain in the winter and the vaccine is basically useless 😬 I remember that happened a few years ago