The position put forward in the BMJ paper is, as per your comment,
> In general because your immune system has a finite memory for diseases and because
and then leans in hard on the erosion and depletion of that finite memory held within the T cell population SARS-CoV-2 is linked to “an unusually high level of ‘indiscriminate’ killing of T cells,”6 says Leitner, adding that this observation is “reminiscent of” measles, which can cause immune amnesia by depleting memory B cells (a different type of immune cell), leaving people vulnerable to pathogens they were previously immune to.
Brazilian researchers found that covid-19 triggered a sharp rise in T cell exhaustion and cellular ageing.10 Although the comparator group was limited, the strongest effects were seen in CD8+ T cells, which suppress latent viruses such as EBV and VZV. These effects were seen even after mild infections.
rather more than > diseases tend to mutate faster when infecting larger number of people
Regardless, all paths land on > its more like the latter.
( > your immune system .. tends to be depleted like your blood sugar. )