If they had been tested at a time when the prevalence was twice as high, then there would be twice as many cases in the vaccinated group, and twice as many cases in the control group. The percentage would be the same.
LS Guy said this: That’s the famous herd immunity. You don’t not catch it because you’re immune; you do not catch it because it’s just not out there to be caught.
Ok, it’s not there to be caught. Regardless, am I immune or not? If so then isn’t that one of the reasons I don’t catch it?
No, that would mean the vaccine was useless. If there was higher prevalance you would expect that there would be a greater increment in the placebo group, even if in absolute percentage the purported efficacy drops.
Don’t get hung up on the word “immunity”. It’s called “immunity”, not “invincibility” for a reason.
“Herd immunity” is a term of art that says the herd collectively is (mostly) immune because most members of the herd will never encounter the infectious agents. They are effectively safe from infection (=“immune”) but not because of any immunity on or inside their own skin. The members of the herd could all be totally non-immune individually and yet the herd as a whole still possesses herd immunity.
I think I’m hung up on the concept. Is immunity not an absolute? Is it more of a mathematical likelihood of avoiding an infection? Can someone determine if I am immune from a disease by examining me? Or am I “immune” because I had the inoculation and it’s extremely unlikely that anyone who had the vaccine ever gets the disease?
I’m not an infectious disease expert, but from my understanding, herd immunity means that enough people are immune that any outbreaks will die off; that is, the R0 is much less than one. So, an individual might get the coronavirus (or measles, or whatever) and they might give it to someone else, but it won’t go much farther because the virus will encounter too many immune people to continue spreading.
I would replace the bolded immune with protected. Herd immunity suggests that the whole herd is mostly protected because if enough herd members are truly immune there will no longer be enough circulating contagion to transmit throughout the herd. So even people who are not immune are now protected.
I just posted this to another thread, it is an excellent article written pre-vaccine. Very readable.
Thanks for posting that. It is a really interesting read.