Some diseases become endemic and pervasive in childhood and end up not causing issues in the elderly who already built up their immune system in childhood. Other times an exposure in childhood ends up cross reacting or something and makes things more deadly later on (I think that's one theory for the 1918 flu; elderly and middle age were protected from an earlier flu exposure, 20-30 year olds had exposure in childhood to something different with a shared antigen that ended up making the immune system overreactive and made things more deadly).

Yes, but unless herd immunity lasts only a couple weeks that shouldn’t be a factor in these Italian analyses. The infection spread over a short period.

I don't know why this comment is being voted down when it is, in fact, correct. Though "temporary" is on a scale proportional to a lifespan.

If an epidemic sweeps through and infects most of the "herd", that herd has herd immunity.

In time as individuals born and others die, the fraction of the herd that is immune goes down and the herd is vulnerable to the same epidemic.

But with vaccination, herd immunity can be sustained indefinitely. With no further epidemics. But, as measles has shown, only as long as a high enough fraction of the herd vaccinates.

Of course it is also possible that if enough of the herd is immune for long enough that the epidemic goes extinct. This takes a lot of work but is how we got rid of Smallpox

Effective contact tracing and social distancing can achieve the same end, which is how we got rid of SARS.