It's a genetic luck of the draw. Here is some explanation:
Genetic or immune defects may impair ability to fight Covid-19.
Genetic or immune defects may impair ability to fight Covid-19. A significant proportion of patients who develop life-threatening forms of Covid-19 have genetic or immunological defects that impair their ability to fight the virus, research has found. The
Covid Human Genetic Effort international consortium describes two glitches in severely ill Covid-19 patients that prevent them from making a frontline immune molecule called type 1 interferon. The patients would have carried these glitches for years before the pandemic, or in the case of the genetic errors, all their lives. The discovery may help to explain a mystery surrounding the coronavirus: why it leaves some sufferers sick or dying in intensive care, while others remain barely affected or asymptomatic.
Covid-19 scientists flag key immune function as a turning point in life threatening cases.
Genetic analysis of Covid-19 patients published in the same journal revealed two dozen gene mutations that had been “silent” until patients were infected by SARS-CoV-2. Researchers -- many of them also involved in the antibody study -- sequenced the genomes of 659 patients with life-threatening cases of the disease; 3.5% carried genetic variations that inhibit interferon production.
Those genetic flaws were similar to the ones that Hoischen and his colleagues from a dozen Dutch centers described in the Journal of the American Medical Association two months ago. The two sets of brothers had inherited a gene mutation that impaired the interferon response, keeping their immune systems from fighting the coronavirus until it had replicated for days.
You don't get to choose your parents.