Probably not what you were looking for as an answer, but one is that, sort of yes, youngsters are bordering on immune. Here I am referring to the finding that has come out of close examination of the "geographic genome tree" of Covid 19. I first came across news of this "tree" having been put together in March and it has "grown" since. It seems it is possible to trace the virus' spread practically down to who infected whom, given access to samples from the different individuals. What then came out was that there was no evidence (within this study) of children up to higher levels of primary school age infecting adults or each other but several examples of adults infecting children. This together with (almost all) infected children showing at most mild symptoms puts them at (not quite) invincible.