How rational individual decisions can make the pandemic worse, and why a good response is hard to maintain

That is the topic of my latest Bloomberg column, here is one excerpt:

The more time passes, the more I wonder if I have, in fact, contracted an asymptomatic version of Covid. The chance of that was quite small in February, but as each month passes it becomes modestly more likely. That realization could easily nudge many people into taking just a bit more risk.

Another train of thought considers the possibility of having a pre-existing protective immune response, perhaps from T-Cells. Experts are not sure of the likelihood or magnitude of this effect, but some have suggested that as many as one-third of Americans may have some built-in protection.

Again, as the months pass, it’s rational for me to upgrade the probability that I have such a protective immune response. With the passage of time, I will feel more protected than I used to.

The basic reasoning is straightforward: Since I haven’t caught a bad form of it by now, I must be relatively safe. Many Americans may or may not grasp the finer points of the immunology and the Bayesian statistical reasoning, but that is a very common-sense kind of response.

And so such people will take more risk — to the detriment of the broader community.

There is much more at the link, including a discussion of intertemporal substitution.  These are some reasons why initially good (or bad) Covid responses tend to get worse, relevant for Europe as well.