I have no idea of the new speak definition, but it appears to be the peak point of protection against acquiring the disease, certainly not a lasting one.
Prior to covid the definition was the lasting level of prevention from catching the disease, often whole life protection from one dose. Not the rapidly fading protection of the Covid vaccines requiring a booster every six months at least.
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To clarify - I'm not referring to how the efficacy is fading over time, lets just pretend that's not happening for the moment for the sake of my question.
When they first announced the vaccines, and the news/government etc would come out with something like "The vaccines are 95% effective", or similar words - what exactly did that mean? How do they work it out? That's the question I'm asking.
e.g.
If a packet of sluggit or something says "reduces your slug population by 95%", then if I used to have 1000 slugs in the garden, I'd expect to have killed 950 of them if I used the sluggit, and so I'd have 50 left. I could then do a risk/benefit evaluation and try and decide if reducing the risk of my lettuces being eaten, is worth potentially killing some innocent wildlife.
If I read that you are 95% less likely to die in a 70mph car crash if you were wearing a seatbelt, Perhaps they had looked at car incident statistics, and found that it would be something like for every 200 car crashes where people weren't wearing seatbelts, 100 died, but for every 200 car crashes where they WERE wearing seatbelts, only 5 died. So they call that (100-5)=95 , therefore seatbelts are 95% effective.
(I'm making those figures up - but thinking of what I've written above in general principle, is that a sensible way of coming up with an efficacy figure? do they do it that way?)
I assume from the talk here, others here are a bit more clued up on this than I am and have looked into it in more depth.
So how do they calculate the Covid vaccine efficacy number? and what does "95% effective" actually mean??