VDR and COVID-19: Consequently, the f VDR isoform allows more synthesis of 25(OH) D, which may partially explain the observed higher vitamin D serum levels in FokI homozygous ff genotype.16,45 Together with our findings, it is plausible to speculate that the VDR Fok I FF genotype, being associated with lower serum 25(OH) D levels, may constitute an independent risk factor for susceptibility to COVID-19.