Given the extensive body of evidence showing that Covid infection causes multi-systemic damage, notably including but not limited to neurological and cardio-vascular, it’s unsurprising that repeat infections rack up ever more damage, with a corresponding increase in Long Covid risk.
And given the solidly established fact that Covid damages the immune system, neither the brevity of infection-acquired immunity nor the fact that prior Covid infections dramatically increase the risk of a whole range of adverse outcomes on every subsequent infection can reasonably be called unexpected.