[usrusr]

Countries that made it well through the first wave suffer from underestimating the threat/overestimating their countermeasures in the second wave. Herd immunity has nothing to do with it, as evidenced by massive second waves in countries like Sweden or Belgium that didn't quite skip the first.

The underestimating/overestimating is actually quite easy to understand: everybody thinks to themselves that a slightly reduced amount of countermeasures should be sufficient compared to the first wave, because it's less of an unknown now. And they might even be right about it! But the same process happens on multiple levels in parallel while everybody assumes all other remaining equal and then they compound. Relative to countermeasures in the first wave, governments weaken executive orders this time (by an amount that might be appropriate all other parameters being equal), regional administrations weaken their execution (by an amount that might be appropriate all other parameters being equal), private organizations, from corporations to knitting clubs weaken countermeasures in their procedures (by an amount that might be appropriate all other parameters being equal) and individual weaken both their personal protection habits and their adherence to all those rules, by an amount that might be appropriate all other parameters being equal.

Nobody can revert all those compounding weakenings, the only single point of correcting for them is at the top level: in the second wave, you need considerably stricter government rules to get the same effect. And even they are only corrected in a reactive way, when the numbers become too bad, when the wave becomes too high. Hence the almost universal big second waves.