To prevent the original strains from coming back when immunity to those (eventually) wanes.

We don't want to start going backwards. Plus, there's cross-reactive immunity so that similar mutations can be recognised by the immune system without ever seeing that particular one before.

Keeping a wide breadth of spike mutations allows that to work more effectively.

Edit: u/nomnomnomnomRABIES, the reason is that Flu is an entirely different beast to COVID. Despite all the mutations COVID has gone through, it is not all that different to the original strain (which is a good reason why our immunity still holds so well). Coronaviruses do not mutate all that much, as they have the largest genome of all RNA viruses. COVID is just mutating a bunch, relatively, because of how widespread it is.

Flu, on the other hand, drifts massively, and constantly. There's no point including older strains because it doesn't help you fend off next year's Flu. Maybe once or twice in your life you'll come across a strain that is similar to one you were previously vaccinated against, which is nice, but no point wasting time cramming a Flu vaccine with all these historical Flu strains.

Yes, and no.

It is less severe because everyone has some kind of immunity to it. The virus inherently is probably still as virulent as the original type to someone with zero immunity.

Wrong.

It doesn't matter if the host dies or not, all that matters is that it can spread before the host dies. COVID was perfectly capable of doing this via asymptomatic spread. (see: Delta).

Omicron outcompeted Delta because it had mutated so wildly that it could bypass all the antibodies the vaccines had generated up to that point, plus it drastically reduced the incubation time, meaning more spread potential. That's it. It could've been as lethal as Delta and would've still been successful.

Omicron is likely as severe as the original strain, the difference is now everyone has some kind of immunity to it, so it wasn't killing people nearly as much on a per-person basis.

> Very cool write up, especially learning how viruses tend to become less lethal and more contagious.

They don't. It's a myth that continues to be propagated because it sounds logical to the layman. It is our immunity that makes them less lethal (when we survive).

If viruses behaved this way as a given, we wouldn't have been getting killed by smallpox and many other viruses for millennia.

You should still get the bivalent booster now if you can. In the US, it is based off of BA.5, which isn't too far removed from the current circulating XBB 1.5 and BQ1.1 strains.

It is likely that later this year we'll get an updated booster again, possibly targeting XBB if it still sticks around.

> To help explain what "inconclusive" means, similar studies were able to conclusively link a 1 in 20,000 risk of narcolepsy to a specific vaccine. So presumably if there's any risk it's lower than 1 in 20,000.

Can you elaborate on why then more recent research has shown that it seems H1N1 was causing narcolepsy itself? Countries that did not use that specific vaccine were seeing surges of narcolepsy incidents as well. Is this not a case of awareness bias, or whatever it is called? How can we be sure it was purely the vaccine?

I thought exercise in old people decreases the risk of stroke?