While I 100% agree with @DrTomFriden's skepticism about herd immunity kicking in at 20-30%, the logic of this statement is wrong. You can have a >50% attack rate if herd immunity kicks in at 30%, here is why....(1/5) https://twitter.com/Atul_Gawande/status/1299318648402780160

When epidemiologists say there is #HerdProtection they mean that the reproductive rate, R, is less than or equal to 1, so cases will start. going down. So R=1 at the peak of an epidemic...(2/5)

If the herd protection threshold is 30%, that means that about 30% of the population will have been infected at this point...and even though each of those people will be infect less than one person, there is still a lot of infection to do.... (3/5)

...and while epidemics are not necessarily symmetric, that you will see as mean cases on the way down as the way up is not a bad approximation. Giving you an attack rate of 60%, even do you had herd immunity at 30%. (4/5)

This is why strategies that let #COVID19 go mostly uncontrolled to rush to herd immunity are #BadIdeas. Uou can infect 2x as many as you have to to have herd protection. The goal is to have as few people infected as possible when we R=1. The best way to do that #Vaccine (5/5)