text: the US National Institutes of Health state there is insufficient evidence to recommend for or against using vitamin D supplementation to prevent or treat COVID-19. The UK National Institute for Health and Care Excellence (NICE) does not recommend to offer a vitamin D supplement to people solely to prevent or treat COVID-19. Both organizations included recommendations to continue the previous established recommendations on vitamin D supplementation for other reasons, such as bone and muscle health, as applicable. Both organizations noted that more people may require supplementation due to lower amounts of sun exposure during the pandemic.
  Several systematic reviews and meta-analyses of multiple studies have described the associations of vitamin D deficiency with adverse outcomes in COVID-19. In the largest analysis, with data from 76 observational studies including almost two million adults, vitamin D deficiency or insufficiency significantly increased the susceptibility to becoming infected with COVID-19 and having severe COVID-19, with odds ratios of 1.5 and 1.9 respectively, but these findings had high risk of bias and heterogeneity. A two-fold greater mortality was found, but this analysis was less robust. These findings confirm smaller, earlier analyses, one of which, in reporting that people with COVID-19 tend to have lower 25(OH)D levels than healthy subjects, stated that the trend for associations with health outcomes was limited by the low quality of the studies and by the possibility of reverse causality mechanisms.
  A meta-analysis of three studies on the effect of oral vitamin D or calcifediol supplementation indicated a lower intensive care unit (ICU) admission rate (odds ratio: 0.36) compared to those without supplementation, but without a change in mortality. A Cochrane review, also of three studies, found the evidence for the effectiveness of vitamin D supplementation for the treatment of COVID-19 to be very uncertain. They found there was substantial clinical and methodological heterogeneity in the three studies that were included, mainly because of different supplementation strategies, vitamin D formulations (one using calcifediol), pre-treatment status and reported outcomes. Another meta-analysis stated that the use of high doses of vitamin D in people with COVID-19 is not based on solid evidence although calcifediol supplementation may have a protective effect on ICU admissions.