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New research from Trinity College Dublin and the University of Edinburgh has examined the association between vitamin D and Covid-19, and found that ambient ultraviolet B (UVB) radiation (which is key for vitamin D production in the skin) at an individual’s place of residence in the weeks before Covid-19 infection was strongly protective against severe disease and death. The paper was published in the journal Scientific Reports recently. Previous studies have linked vitamin D deficiency with an increased susceptibility to viral and bacterial respiratory infections. Similarly, several observational studies found a strong correlation between vitamin D deficiency and Covid-19, but it could be that these effects are confounded and are in fact a result of other factors, such as obesity, older age or chronic illness, which are also linked with low vitamin D, said the authors.

To overcome this, researchers were able to calculate a “genetically-predicted” vitamin D level, which is not confounded by other demographic, health and lifestyle factors, by using the information from over 100 genes that determine vitamin D status.

The Mendelian Randomisation is a particular analytical approach that enabled researchers to investigate whether vitamin D and Covid-19 might be causally linked using genetic data. Few earlier studies attempted this, but failed to show a causal link. This could be because UVB radiation sunshine, which is the most important source of vitamin D for the majority of people, was ignored.

Researchers, for the first time, looked jointly at genetically-predicted and UVB-predicted vitamin D levels. Almost half a million individuals in the UK took part in the study, and ambient UVB radiation before Covid-19 infection was individually assessed for each participant. When comparing the two variables, researchers found that correlation with measured vitamin D concentration in the circulation was three-fold stronger for UVB-predicted vitamin D level, compared to genetically-predicted.

Researchers found that ambient UVB radiation at an individual’s place of residence preceding Covid-19 infection was strongly and inversely associated with hospitalisation and death. This suggests that vitamin D may protect against severe Covid-19 disease and death.

Additionally, while the results from the Mendelian Randomisation analysis were not conclusive, some indication of a potential causal effect was noted. Because of the relatively weak link between genetically-predicted vitamin D level that is used for Mendelian Randomisation analysis, it is possible that the number of cases in the current study was too small to convincingly determine causal effect, but future larger studies might provide the answer.

Prof Lina Zgaga, Associate Professor in Epidemiology, School of Medicine, Trinity College, and senior researcher on the study, said: “Our study adds further evidence that vitamin D might protect against severe Covid-19 infection. Conducting a properly designed Covid-19 randomised, controlled trial of vitamin D supplementation is critical. Until then, given that vitamin D supplements are safe and cheap, it is definitely advisable to take supplements and protect against vitamin D deficiency, particularly with winter on the horizon.”