Thread by @andrewflood

A 'how can we have a new wave with 75% of the population vaccinated' thread. Short answer is Delta is a nightmare, way, way nastier than old school Covid19 because one person on average will infect 6 with no measures in place rather than 3. /1

Imagine a 100% effective vaccine that stopped transmission completely. With old school Covid once 2 in 3 are vaccinated then the virus finds 2 of its 3 previously successful jumps to other people blocked. Above 66% vaccination 1000 cases only become 900, 750, 580 etc /2

We don't know for sure but it looks like Delta infects 6 other people without measures in place. To reduce that to 1 person 5 of those would have to be blocked by 100% effective vaccine. 5/6 is 83%. So at our 75% vaccination Delta is still getting to more than 1 person /3

We also don't have a vaccine thats 100% effective. In fact the reason most sensible people gave up talking abut herd immunity is that the math made clear in July that with Delta the concept was meaningless. Vaccination would slow the pandemic - very important but not stop it /4

So plan B is that the vaccines greatly reduce the percentage of cases that get hospitalised or go to ICU. The pressure on the health service gets cut if everyone is vaccinated to a fraction of the previous level. Thats the whole 'living with the virus' / endemic bit /5

Problem is even with 100% vaccination a lot of people get hospitalised each year if the vaccine is say 95% effective at stopping hospitalisation. And if 10% don't get vaccinated well then a lot of them get hospitalised too & they also infect more people. /6

So the other bit of 'living with the virus' is hoping that the unvaccinated get infected & mostly survive at a rate that doesn't overwhelm healthcare. And that infection gives them sufficient immunity against future hospitalisations that the problem reduces over times /7

But we know all forms of immunity against infection wane. So we will keep seeing waves of infection - the huge but unknown question is what proportion of 3rd or 4th infections etc get hospitalised. If that stays high (say 3 per 1000) we are in a lot of trouble /8

The pandemic has been characterised by people who claimed to have exact answers to the unknowns above. Most useful experts say 'we don't yet know, 'we need to research that' there isn't enough data yet' - which means we need to be careful about risk and be willing to reverse

The above is an assemblage of my understanding based of listening to actual experts, which I've then simplified to make it easy to explain (eg no herd immunity threshold formulas). So open to correction if I've over done that - Think in terms of ballpark its correct though