Vitamin D has a special position among the vitamins, as it is both supplied through diet and synthesized by humans themselves through exposure to UVB light (solar radiation). 

Intake through the diet with the usual foods alone is not sufficient to achieve target blood serum levels of 25 (OH) D* of at least 50 nmol/l (20ng/mL), as recommended by EFSA, DGE and many other official societies. The difference must be covered by endogenous synthesis (frequent sun exposure) or by taking a vitamin D supplementation. 

But even today serum 25(OH)D levels below 50 nmol/l are widespread. An epidemiological study conducted in Germany in 2015 on adults 18–79 years, showed that between June and September only half of women had levels ≥ 50 nmol/l, and only half of men between June and October. Even worse: 25 % of men had serum 25(OH)D levels <30 nmol/l from November to April, 25 % of the women from November to May**.  

2016, a research group comes to the conclusion that in Europe ‘Vitamin D deficiency is evident throughout the European population at prevalence rates that are concerning’. 13% of the European population have serum 25(OH)D concentrations <30 nmol/L on average in the year, with 18% in winter time.*** 

More than 10 observational studies reveal surprisingly clear correlations between 25(OH) D levels and Covid-19 risks. The lower the vitamin D levels in the blood, the higher the risks of severe infections and mortality. And some statistical data indicate, that the incidence of Covid-19 infections in populations with better 25 (OH) D levels was lower.  The attached publication issued October 31, 2020 reviews the evidence available so far. 

The British National Institute for Care and health Excellence (NICE) published in June 2020 recommendations re vitamin D supplementation in view of the COVID-19 pandemic.

They advise that “all people should consider taking a daily supplement containing 10 micrograms vitamin D during autumn and winter months”. ****

*: 25-hydroxyvitamin D = 25(OH)D = calcidiol

**:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4499202/

***:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5527850/

****:https://www.nice.org.uk/advice/es28/resources/covid19-rapid-evidence-summary-vitamin-d-for-covid19-pdf-1158182526661