Originally Posted by
Loren Pechtel
Huh? Herd immunity doesn't require people to be completely immune, it merely requires that it spread poorly enough that R0 is below 1. Look at the current situation--the R0 of Delta is about 6-7. A few posts above this one we have data that vaccination provides 6-7x lower infection rates. By itself, that puts the R0 at about 1--but breakthrough cases are generally shorter than regular cases, thus reducing the exposure. Thus we should already have a R0 somewhat below 1 even now--herd immunity amongst the vaxxed but continual introduction from the unvaxxed causing it to persist.
Definitiion of herd immunity have shifted around somewhat. The original version is from farming. Imagine you are a farmer and you need to vaccinate your cattle or sheep against something like pasteurella, a respiratory disesase similar to pneumonia. You have hundreds of cattle spread over a huge geography (e.g. Australia) or a thousand sheep scattered around a mountain (e.g. upland UK). You have a dog or two, a quad bike, some fencing and that's it. Now typically you can round up 80% of the animals in 30 minutes, the next 10% will take an hour, and it will be almost impossible to round up all the livestock quickly in summer. But that's ok, since if you round up and then vaccinate 85% of your livestock you will reach herd immunity for that particular field. The bacteria (in this case) won't transmit very well in that field since it will encounter mostly vaccinated animals and that vaccine is pretty effective. Consequently a farmer makes a deliberate choice not to go mad rounding up the sheep, he also knows that mass vaccination at speed makes a few mistakes (so the dose isn't properly administered), again that's OK.
In this pandemic decision makers have speculated that once vaccination reaches a particular percentage of the population then COVID will lose its power in some definition or other. Delta has clearly made that very difficult, and certainly pushed up the percentage, due to a combination of higher transmission plus the fact that a minority of those vaccinated can and are getting Delta, albeit less seriously, as well as potentially transmitting it too - so go back to the field and you see the issue, you would need a higher percentage than you first thought and arguably more regular vaccination of at least the more vulnerable animals. In the case of COVID it may well be that herd immunity is impossible or so difficult that the concept is unhelpful.
Personally I think this is overblown, we're not in a field, it's a global society and the virus is evolving very quickly. It's tempting to look for zero sum outcomes, or a magic vaccination figure but that's not really on the cards at the moment. The herd immunity usage in farming is for very well known and established diseases.