It is well known that the severity of a covid-19 infection is directly related to the dose of virus that the patient encountered. A short encounter will seldom land you in the hospital. But having been in a small room for half an hour with somebody who is infected carries a significant risk.
The question that is almost never addressed is what happens with all those people who get only very lightly infected. It is implicitly assumed that they don't build any resistance and will stay at risk of being infected.
I don't believe this. I consider it much more likely that the immune system each time builds a little bit of resistance. It may not be enough to make you immune at once. But very likely you will become less sick when you are infected with a heavy dose of the virus later on.
This makes the "herd immunity" discussion missing the point. Sure, you want that enough people to build immunity. But you want it to happen gradually. This may even happen one virus at a time.
We can see this light immunity already in action. In most areas covid-19 is nowadays much less deadly than a few months ago.
This is what naturally happens with influenza and corona viruses. Many people never get sick and yet after one or two years the virus disappears. Obviously they have built some kind of immunity.
One group that doesn't get this are the youth holding corona parties. Instead of building immunity with low doses they take a high dose at once - and take a considerable risk.
The other group that doesn't get it are the countries that impose a heavy lockdown. Yes, you may drive down the number of infections that way. But as everyone is locked up they don't get the light infections either and they don't build resistance. So those countries carry the highest risk for a second wave.
It is Sweden that has understood this model the best. They underestimated the initial deadliness of the disease and as a consequence initially provided insufficient protection to vulnerable groups. But they were right to put the focus on stopping mass meetings and other situations where heavy infections were most likely to occur while simultaneously allowing lower intensity contacts to proceed normally.