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)
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)
