A recent editorial in the BMJ referred to two published RCTs.1 Cross-sectional studies report a link between low serum vitamin D levels and catching covid-19. One trial in the UK compared vitamin D at either 3200 IU/day vs 800 IU/day for six months and found no difference in the prevalence of covid-19. 65% of participants had less than 75 nmol/L of blood vitamin D. The other was in Norway, where low dose vitamin D was given vs placebo, and no difference was seen.
The authors suggest not giving vitamin D to those with normal vitamin D levels, instead focusing on higher-risk people such as pregnant women, the elderly and those with less exposure to the sun, and supplement 1000-2000 IU/day if they have low blood levels of vitamin D. The standard NZ cholecalciferol contains 50,000 IU of vitamin D.