On the reinfection fallacy
You might have also heard of people becoming reinfected by SARS-CoV-2. Indeed, immunity, natural or vaccine-induced, isn’t the impenetrable shield described by many. Essentially harmless and asymptomatic reinfections do take place. That is, in fact, the very mechanism by which adaptive immunity is triggered.
However, symptomatic reinfections are very rare. Like an army which adapts its response to the size and the progression of its enemy forces, adaptive immunity provides a specific, rapid and resource-optimised response. As such, reinfections are mostly asymptomatic and recovered patients are protected from severe disease.
In fact, innocuous reinfections can play a positive public health role as continuous immune updates of the population. They can help a seamless and progressive adaptation to emerging new variants and strains. And indeed a recent study showed that couples with children were more frequently asymptomatic than couples without, most likely because children acted as natural and harmless immunisation vehicles. The likely reason high density countries all have very low death tolls is that they have asymptomatic reinfections that regularly and widely update the immunity of the population.