With delta, it probably isn't possible to get herd immunity by vaccinating even 100% of the population (not that they're eligible anyway) - it just isn't effect

With delta, it probably isn't possible to get herd immunity by vaccinating even 100% of the population (not that they're eligible anyway) - it just isn't effective enough at preventing transmission. The maths on this is actually pretty easy: the delta variant has an R0 of somewhere about 8, meaning each person infects around 8 others, latest numbers from the UK suggest vaccinated people are about a third as likely to have it as unvaccinated but are probably just as infectious, vaccinate everyone and each infected person will still spread it to enough others that there'll be strong exponential growth. There's also going to be diminishing returns on vaccinating the last few percent of the population. At some point, the vaccinated outnumber the unvaccinated so much that the large majority of cases, hospitalisations, deaths, and spreading of the virus is happening amongst the vaccinated. Then there's all the people who aren't eligible for vaccination at all, of course...