I'm sure there's genetic variability in the effectiveness of the immune systems (plural as I see the antigen and T-cell systems as quite different) but there's another genetic variability: There are single nucleotide polymorphisms that affect the ACE2 receptor - some make it more difficult for the Covid-19 spike proteins to 'latch on'. Having these don't make you invulnerable to the disease, but it takes a higher viral load to become infected. Good overview here.