It is posited by contributors      to BMJ, 'Preventing a covid-19 pandemic', others,    and myself, that vitamin D, and particularly vitamin D deficiency, likely factors in the progression, and/ or severity, and/ or mortality of COVID-19; and may present clinical treatment opportunities.
My preprint, 'Vitamin D deficiency: a factor in COVID-19, progression, severity and mortality? – An urgent call for research',9 posits, that greater vitamin D deficiency <25nmol/L in southern (E.g. Spain and Italy), than northern European Countries (E.g. Germany, Norway, Finland, Iceland), may help account for their differentials, in mortality rates per million. Consistent with this, Northern Europeans have higher vitamin D food intakes, food fortification, and supplement more. This is an easily testable posit.
Northerly resident; Europeans with darker skins; BAME, and African Americans; as well as more southerly elderly Spanish and Italians; are often vitamin D deficient. IF, vitamin D deficiency, increases the risk of COVID-19 related; infection, hospitalisation and mortality; one would expect, and indeed sees, higher COVID-19 hospitalisation and mortality, in; dark-skinned (Fitzpatrick scale) northerly residents; those with dress codes that inhibit sunlight; and generally in groups likely to include the 'D' deficient.
Whilst data is still sparse, it supports the above contention; albeit lack of distancing, and vitamin D deficiency, may both be contributing factors: “40 % of the reported COVID-19 related deaths occurring in Stockholm involve the Somali diaspora communities”; “18 % (estimated) of the COVID-19 deaths country-wide are from the Somali community,”   (Numbers of Somalis in 2016 in Stockholm municipality approx. 7,827 of 935,619 persons = 0.84%)(Metropolitan Stockholm 2.4M (2016)) (Total Swedish population 9.5M – of which approx. 63,853 were Somalis = 0.67% (2016)) 
Data clearly shows Somalis, and wider immigrant groups, are D deficient, for example, “Vitamin D deficiency ( < 25 nmol/l) was found in 73% of the Somali women and in 1% of the controls” (Sweden); “according to several studies, certain categories of immigrants living in the northern part of Europe are vitamin D deficient “; “A significantly higher proportion of immigrant women (77.9%) had levels < 25 nmol/l compared with 3.9% in controls; 29.4% had 25-hydroxyvitamin D levels, < 12 nmol/l, but none of the controls.”(Sweden) “A high prevalence of vitamin D insufficiency (S-25OHD < 50 nmol/l) was observed (89·6 %) in the Somali group.” (Finland) 
In respect of African Americans, a headline, April 3rd, 2020, 'Early Data Shows African Americans Have Contracted and Died of Coronavirus at an Alarming Rate', observes, “African Americans made up almost half of Milwaukee County's 945 cases and 81% of its 27 deaths in a county whose population is 26% black.”; and “In Michigan, where the state's population is 14% black, African Americans made up 35% of cases and 40% of deaths as of Friday morning. Detroit, where a majority of residents are black, has emerged as a hot spot with a high death toll. As has New Orleans. Louisiana has not published case breakdowns by race, but 40% of the state's deaths have happened in Orleans Parish, where the majority of residents are black.”; “Illinois and North Carolina are two of the few areas publishing statistics on COVID-19 cases by race, and their data shows a disproportionate number of African Americans were infected.”17
WBEZ News reports, April 5th, “In Chicago, 70% of COVID-19 Deaths Are Black,” in Cook County, “While black residents make up only 23% of the population in the county, they account for 58% of the COVID-19 deaths. And half of the deceased lived in Chicago, according to data from the Cook County Medical Examiner's office”.
The review, APHA 'Call for Education and Research Into Vitamin D Deficiency/Insufficiency', in 2008, noted, “Healthy men and women older than the age of 65 in Boston were surveyed for vitamin D status, which revealed that a surprising 34% of White, 42% of Hispanic, and 84% of African American men and women were vitamin D deficient.” (Definitions of 'D' deficiency vary).
“An estimated 40% of American adults may be vitamin D deficient. For African-Americans, that number may be nearly double at 76% according to a new study by The Cooper Institute.” (defined as <30nmol/L); it is also noted they “have higher rates of and obesity”.  Risk is also higher in adolescents, NHANES III noted, “non-Hispanic black adolescents had 20 times the risk of serum 25-hydroxyvitamin D <20 ng/mL compared with non-Hispanic white adolescents. Risk was more than double for females compared with males.” There was an inverse relationship with weight.
In contrast, in continental Africa, whilst Vitamin D deficiency exists, it does not appear to be as widespread as in the United States “one in five people living in Africa had a low 25(OH)D concentration with use of a less than 30 nmol/L.” COVID-19 data for Somalia is limited. Test are done externally, as no there is no testing facility in the country: current data; 7 cases, 1 recovered, no deaths. The first recorded case was confirmed on the 16th of March. Twenty volunteer doctors from Somali National University went to Italy to help fight the outbreak there.  Case numbers in wider Africa are currently relatively low, compared to the most infected countries, but arguably there is not sufficient data, or certainty, as to initial infection dates, to, at this point, even postulate as to future COVID-19 infection, or mortality rates, in Africa.
Differences appear to exist in the metabolism of Vitamin D; 'The D paradox'; between Caucasians and African Americans, particularly in terms of bone density. However, the metabolism of vitamin D is complex, and pathways impacting COVID-19, likely differ from those regulating bone densities.
It is well recognised COVID-19 mortality rate, has very sadly, been high in Italy (15,887 as at 5TH April 2020). A letter to the BMJ titled 'Preventing a covid-19 pandemic Can high prevalence of severe hypovitaminosis D play a role in the high impact of Covid infection in Italy?' notes, “A study from Isaia et al on 700 women aged 60-80 yrs in Italy found values of 25OH vitamin D lower than 5 ng/ml in 27% of the women and lower than 12 ng/ml in as many as 76%.” “Finally, another Italian study found a winter prevalence of hypovitaminosis D up to 32% of healthy postmenopausal women and to 82% in patients engaged in long-term rehabilitation programmes because of various neurological disorders.”29
BAME (Black, Asian, Minority Ethnic) persons in the United Kingdom, may be more susceptible. A study of 2,249 patients by the UK 'Intensive Care National Audit and Research Centre', noted “Despite making up just 13 per cent of the UK population, a third of patients who fall critically ill with COVID-19 are from black, Asian or minority ethnic (BME) groups.” Further, sadly, 6 out of 8, UK COVID-19 medical staff deaths, were British Muslims. Hasidic Jews in Israel may also be at higher COVID-19 risk, due to failure to isolate and/ or greater risk of vitamin D deficiency. 
I agree with Professor Cobbold's earlier comment in this BMJ thread, “To me the dots are joined up and the picture points to an unexploited potential emergency therapy for covid-19, especially in the elderly who are typically severely D3 deficient by physiological criteria, and at highest risk.”,2 and to which low 'D' risk group more could be added; those with darker skin tones in more northerly and southerly latitudes; those with high clothing coverage; women using UVB blockers; those with limited access to sun including; people in institutions; those working shifts; those in naval ships and submarines; those in institutions; the obese; and indeed all at risk of vitamin D deficiency.
Arguably it is urgent, that research is done, to determine if vitamin D deficiency factors in COVID-19, infection, progression, severity and mortality. Vitamin D blood spot tests are cheap. Patient records have to be maintained in any event. The additional work, and risk burden, in taking vitamin D measurements are limited. However, the reward could be very significant, even a 'game changer'.
Further, any determination that vitamin D factors in COVID-19, incidence progression and outcome, would point to the likelihood of COVID-19 following a seasonal pattern in populations, which would require factoring into determination of Governmental COVID-19 forward planning policies; mortality risk modelling; social distancing polices; and population vitamin D optimisation, including access to outside spaces, to facilitate sensible, ethnicity appropriate, exposure to UVB in sunlight; and/ or free 'D' supplementation, at least for the most vulnerable and financially insecure.
• Test all COVID-19 patients in hospital/s at a given point in time for Vit D, follow, and report results.
• Take finger prick samples at the same time as COVID-19 test – follow through with laboratory vitamin D tests on the samples of all positives, and an equal number of controls; report results.
• A study using Vit D clinically – test for low vit D – where low supplement with vitamin D3, and/ or as an alternative to above, try a 1,25-active form;  report results.
Vitamin D, as suggested in recent papers, could also usefully be immediately incorporated into wider clinical nutritional COVID-19 protocols,  but the above studies would still be required as a matter of urgency, to better understand the role of vitamin D in COVID-19, for the formulation of; treatment, prevention, and wider, policies.
 Rhein, H. (6th March 2020). BMJ 2020;368:m810 doi: https://doi.org/10.1136/bmj.m810
 Cobbold, P. (2nd March 2020). BMJ 2020;368:m810 doi: https://doi.org/10.1136/bmj.m810
 Maestri, E., Formoso, G., Da Cas R., Mammarella, F,. Trotta, F. (12th March 2020)
Vitamin D against COVID 19: Clinicians need more than observations and hope. BMJ 2020;368:m810 doi: https://doi.org/10.1136/bmj.m810
 Grant, W. (1st April 2020). Can vitamin D supplementation reduce the spread of COVID-19? Try first with health care workers and first responders.: BMJ 2020;368:m810 doi: https://doi.org/10.1136/bmj.m810
 Cobbold, P. (8th March 2020). BMJ 2020;368:m810 doi: https://doi.org/10.1136/bmj.m810
 McCartney, D., Byrne, D. (2020). Optimisation of Vitamin D Status for Enhanced Immuno-protection Against Covid-19. Issue: Ir Med J; Vol 113; No. 4; P58. http://imj.ie/optimisation-of-vitamin-d-status-for-enhanced-immuno-protection-against-covid-19/
 Grant, W., Lahore, H., McDonnell, S., Baggerly, C., French, C., Aliano, J., & Bhattoa, (30th March 2020 2nd version) H. Evidence that Vitamin D Supplementation Could Reduce Risk of Influenza and COVID-19 Infections and Deaths. Nutrients 2020, 12, 988. https://www.preprints.org/manuscript/202003.0235/v2
 Isaia, G. (25th March 2020). Possibile ruolo preventivo e terapeutico della vitamina D nella gestione della pandemia da COVID-19 Enzo Medico, Università degli Studi di Torino, Torino,
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