UK Biobank Mendelian randomization study not finding significant differences in COVID-19 risk. The number of people predicted to have vitamin D deficiency does not appear to be provided.For some background on Mendelian randomization studies and their limitations see [1].For reasons why Mendelian randomization may fail in this case, see [2]. Authors suggest that it may come down to the use of 25-OHD concentration in serum as a less than ideal proxy for vitamin D status of cells involved in the immune response. For most other purposes, it may not matter much that unbound (free) 25-OHD is the better predictor of vitamin D deficiency and the resulting unfavourable outcomes. But for the MR analysis, the genetic instrument is strongly dominated by variation in the GC gene which modulates the concentration of vitamin D-binding protein (VDBP) in blood and thereby indirectly the concentrations of 25-OHD and 1,25-dihydroxy vitamin D. Thus, the common GC alleles rs4588A and rs7041T are both associated with much lower than average vitamin D concentrations. In contrast, directly measured unbound (free) vitamin D concentrations are minimally affected by these alleles, if at all.