key: cord-0259592-kuvdbr09
authors: Borsche, L.; Glauner, B.; von Mendel, J.
title: COVID-19 mortality risk correlates inversely with vitamin D3 status, and a mortality rate close to zero could theoretically be achieved at 50 ng/ml 25(OH)D3: Results of a systematic review and meta-analysis
date: 2021-09-25
journal: nan
DOI: 10.1101/2021.09.22.21263977
sha: 99cc4b29c4a61bfdaf17c60a6f9362387b628a25
doc_id: 259592
cord_uid: kuvdbr09

Background Much research shows that blood calcidiol (25(OH)D3) levels correlate strongly with SARS-CoV-2 infection severity. There is open discussion regarding whether low D3 is caused by the infection or if deficiency negatively affects immune defense. The aim of this study was to collect further evidence on this topic. Methods Systematic literature search was performed to identify retrospective cohort as well as clinical studies on COVID-19 mortality rates vs. D3 blood levels. Mortality rates from clinical studies were corrected for age, sex and diabetes. Data was analyzed using correlation and linear regression. Results One population study and seven clinical studies were identified, which reported D3 blood levels pre-infection or on the day of hospital admission. They independently showed a negative Pearson correlation of D3 levels and mortality risk (r(17)=-.4154, p=.0770/r(13)=-.4886, p=.0646). For the combined data, median (IQR) D3 levels were 23.2 ng/ml (17.4 - 26.8), and a significant Pearson correlation was observed (r(32)=-.3989, p=.0194). Regression suggested a theoretical point of zero mortality at approximately 50 ng/ml D3. Conclusions The two datasets provide strong evidence that low D3 is a predictor rather than a side effect of the infection. Despite ongoing vaccinations, we recommend raising serum 25(OH)D levels to above 50 ng/ml to prevent or mitigate new outbreaks due to escape mutations or decreasing antibody activity.

One population study and seven clinical studies were identified, which reported D3 blood levels preinfection or on the day of hospital admission. They independently showed a negative Pearson correlation of D3 levels and mortality risk (r(17)=-.4154, p=.0770/r(13)=-.4886, p=.0646). For the combined data, median (IQR) D3 levels were 23.2 ng/ml (17.4 -26.8) , and a significant Pearson correlation was observed (r(32)=-.3989, p=.0194). Regression suggested a theoretical point of zero mortality at approximately 50 ng/ml D3. 

The two datasets provide strong evidence that low D3 is a predictor rather than a side effect of the infection. Despite ongoing vaccinations, we recommend raising serum 25(OH)D levels to above 50 ng/ml to prevent or mitigate new outbreaks due to escape mutations or decreasing antibody activity.

Trial registration Not applicable. 

The SARS CoV 2 pandemic causing acute respiratory distress syndrome (ARDS) has lasted for more -than 18 months. It has created a major global health crisis due to the high number of patients requiring intensive care, and the high death rate has substantially affected everyday life through contact restrictions and lockdowns. According to many scientists and medical professionals, we are far from the end of this disaster and hence must learn to coexist with the virus for several more years, perhaps decades [1, 2] .

It is realistic to assume that there will be new mutations, which are possibly more infectious or more deadly. In the known history of virus infections, we have never faced a similar global spread. Due to the great number of viral genome replications that occur in infected individuals and the error-prone nature of RNA-dependent RNA polymerase, progressive accrual mutations do and will continue to occur [3] [4] [5] .

Thus, similar to other virus infections such as influenza, we have to expect that the effectiveness of vaccination is limited in time, especially with the current vaccines designed to trigger an immunological response against a single viral protein [6] [7] [8] . . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) preprint

The copyright holder for this this version posted  https://doi.org/10.1101/2021.09. 22.21263977 doi: medRxiv preprint We have already learned that even fully vaccinated people can be infected [9] . Currently, most of these infections do not result in hospitalization, especially for young individuals without comorbidities.

However, these infections are the basis for the ongoing dissemination of the virus in a situation where worldwide herd immunity against SARS CoV 2 is rather unlikely. Instead, humanity could be trapped in -an insuperable race between new mutations and new vaccines, with an increasing risk of newly arising mutations becoming resistant to the current vaccines [3, 10, 11] . Thus, a return to normal life in the near future seems unlikely. Mask requirements as well as limitations of public life will likely accompany us for a long time if we are not able to establish additional methods that reduce virus dissemination.

Vaccination is an important part in the fight against SARS CoV 2 but, with respect to the situation -described above, should not be the only focus. One strong pillar in the protection against any type of virus infection is the strength of our immune system [12] . Unfortunately, thus far, this unquestioned basic principle of nature has been more or less neglected by the responsible authorities. It is well known that our modern lifestyle is far from optimal with respect to nutrition, physical fitness and recreation. In particular, many people are not spending enough time outside in the sun, even in summer. The consequence is widespread vitamin D deficiency, which limits the performance of their immune systems, resulting in the increased spread of some preventable diseases of civilization, reduced protection against infections and reduced effectiveness of vaccination [13] .

In this publication, we will demonstrate that vitamin D3 deficiency, which is a well-documented worldwide problem [13] [14] [15] [16] [17] [18] [19] 179] , is one of the main reasons for severe courses of SARS CoV 2 --

infections. The fatality rates correlate well with the findings that elderly people, black people and people with comorbidities show very low vitamin D3 levels [16, [20] [21] [22] . Additionally, with only a few exceptions, we are facing the highest infection rates in the winter months and in northern countries, which are known to suffer from low vitamin D3 levels due to low endogenous sun-triggered vitamin D3

synthesis [23] [24] [25] [26] .

Vitamin D3 was first discovered at the beginning of the 19 th century as an essential factor needed to . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) preprint

The copyright holder for this this version posted September 25, 2021. ; https://doi.org/10.1101/2021.09.22.21263977 doi: medRxiv preprint guarantee skeletal health. This discovery came after a long period of dealing with the dire consequences of rickets, which causes osteomalacia (softening of bones). This disease especially affected children in northern countries, who were deprived of sunlight and often worked in dark production halls during the industrial revolution [27] . At the beginning of the 20 th century, it became clear that sunlight can cure rickets by triggering vitamin D3 synthesis in the skin. Cod liver oil is recognized as a natural source of vitamin D3 [28]. At the time, a blood level of 20 ng/ml was sufficient to stop osteomalacia. This target is still the recommended blood level today, as stated in many official documents [29] . In accordance with many other publications, we will show that this level is considerably too low to guarantee optimal functioning of the human body.

In the late 1920s, Adolf Windaus elucidated the structure of vitamin D3. The metabolic pathway of vitamin D3 (biochemical name cholecalciferol) is shown in Figure 1 [30]. The precursor, 7dehydrocholesterol, is transformed into cholecalciferol in our skin by photoisomerization caused by UV-B exposure (wavelength 280-315 nm). After transportation to the liver, cholecalciferol is hydroxylated, resulting in 25-hydroxycholecalciferol (25(OH)D3, also called calcidiol), which can be stored in fat tissue for several months and is released back into blood circulation when needed. The biologically active form is generated by a further hydroxylation step, resulting in 1,25-dihydroxycholecalciferol (1,25(OH)2D3, also called calcitriol). Early investigations assumed that this transformation takes place mainly in the kidney. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) preprint [40, 41] , diabetes mellitus [42] , acute respiratory tract infections [43] , chronic inflammatory diseases [44] and autoimmune diseases such as multiple sclerosis [45] . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) preprint Receptor binding engages the formation of the "vitamin D3 response element" (VDRE), regulating a large number of target genes involved in the immune response [56] . As a consequence of this knowledge, the scientific community now agrees that calcitriol is much more than a vitamin but rather a highly effective hormone with the same level of importance to human metabolism as other steroid hormones.

The blood level ensuring the reliable effectiveness of vitamin D3 with respect to all its important functions came under discussion again, and it turned out that 40-60 ng/ml is preferable [37], which is considerably above the level required to prevent rickets.

Long before the SARS CoV 2 pandemic, an increasing number of scientific publications showed the -effectiveness of a sufficient vitamin D3 blood level in curing many of the human diseases caused by a weak or unregulated immune system [37, [57] [58] [59] . This includes all types of virus infections [43, [60] [61] [62] [63] [64] [65] [66] [67] [68] 180] , with a main emphasis on lung infections that cause ARDS [69] [70] [71] , as well as autoimmune diseases [45, 62, 72, 73] . However, routine vitamin D3 testing and supplementation are still not established today. Unfortunately, it seems that the new findings about vitamin D3 have not been well accepted in the medical community. Many official recommendations to define vitamin D3 deficiency still stick to the 20 ng/ml established 100 years ago to cure rickets [74] .

Additionally, many recommendations for vitamin D3 supplementation are in the range of 5 to 20 µg per day (200 to 800 international units), which is much too low to guarantee the optimal blood level of 40-60 ng/ml [37, 75] . One reason for these incorrect recommendations turned out to be calculation error [76, 77] .

Another reason for the error is because vitamin D3 treatment to cure osteomalacia was commonly (175) . Angiotensin-converting enzyme 2 (ACE2) is responsible for converting angiotensin II to angiotensin- (1, 7) . Angiotensin II primarily triggers vasoconstriction but can also cause inflammation, fibrosis and oxidative stress in the absence of its counterpart, angiotensin- (1, 7) . ACE2 is the primary receptor of SARS-CoV-2, which decreases its activity, leading to an increase in angiotensin II levels and a decrease in angiotensin- (1, 7) levels. This effect ultimately triggers SARS-CoV-2-induced "acute respiratory distress syndrome" (ARDS) [83, 84] . Calcitriol, the active metabolite of vitamin D3, minimizes this effect by inhibiting renin expression and thus angiotensin II synthesis and by stimulating ACE2 expression [172, 173] , enhancing the conversion of angiotensin II to angiotensin- (1, 7) . Thus, insufficient vitamin D blood levels lead to the development of severe courses of SARS-CoV-2 disease. In addition, it has been shown that high angiotensin II levels lead to downregulation of the enzyme 1-alpha-hydroxylase [174] , which is required for the formation of calcitriol, thereby exacerbating the negative consequences of vitamin D deficiency. Angiotensin converting enzyme 2 (ACE2), a part of the renin angiotensin system (RAS), serves as the -major entry point for SARS CoV 2 into cells (Fig. 2) . When SARS CoV 2 is attached to ACE2 its ---expression is reduced, thus causing lung injury and pneumonia [83, 84, 175] .  Experimental studies have shown that vitamin D and its metabolites modulate endothelial function and vascular permeability via multiple genomic and extragenomic pathways [101, 102] .

 Vitamin D reduces coagulation abnormalities in critically ill COVID-19 patients [103] [104] [105] .

A rapidly increasing number of publications are investigating the vitamin D3 status of SARS CoV 2 -patients and have confirmed both low vitamin D levels in cases of severe courses of infection [106] [107] [108] [109] [110] [111] [112] [113] [114] [115] [116] [117] [118] [119] [120] [121] and positive results of vitamin D3 treatments [122] [123] [124] [125] [126] [127] [128] . Therefore, many scientists recommend vitamin . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) preprint

The copyright holder for this this version posted September 25, 2021. ; https://doi.org/10.1101/2021.09.22.21263977 doi: medRxiv preprint [14, 18, 75, 82, 129, 130] , which has additionally resulted in proposals for the consequent supplementation of the whole population [131] . A comprehensive overview and discussion of the current literature is given in a review by Linda Benskin [132] . Unfortunately, all these studies are based on relatively low numbers of patients. Well-accepted, placebo-controlled, double-blinded studies are still missing.

The finding that most SARS CoV 2 patients admitted to hospitals have vitamin D3 blood levels that are -too low is unquestioned even by opponents of vitamin D supplementation. However, there is an ongoing discussion as to whether we are facing a causal relationship or just a decline in the vitamin D levels caused by the infection itself [82, 133, 134, 181] .

There are reliable data on the average vitamin D3 levels in the population [15, 19, 135] in several countries, in parallel to the data about death rates caused by SARS CoV 2 in these countries [136, 137] .

Obviously, these vitamin D3 data are not affected by SARS CoV 2 infections. While meta-studies using -such data [25, 130, 134, 138] are already available, our goal was to analyze these data in the same manner as selected clinical data. In this article, we identify a vitamin D threshold that virtually eliminates excess values measured post-infection latest on the day after hospitalization. Thus, we can expect that the measured vitamin D status is still close to the preinfection level. In contrast to other meta-studies which also included large retrospective cohort studies [184] [185] , our aim was to perform regressions on the combined data after correcting for patient characteristics.

These results from independent datasets, which include data from before and after the onset of the disease, also further strengthen the assumption of a causal relationship between vitamin D3 blood levels and SARS-CoV-2 death rates. Our results therefore also confirm the importance of establishing vitamin D3 supplementation as a general method to prevent severe courses of SARS CoV 2 infections.

-- is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) preprint 

Collected studies were divided into a population study [143] and seven hospital studies. Notably, these data sources are fundamentally different, as one assesses vitamin D values long-term, whereas the other measures vitamin D values postinfection, thereby masking a possible causal relationship between the preinfection vitamin D level and mortality.

Several corrections for the crude mortality rates (CMRs) recorded by Ahmad were attempted to understand the underlying causes within the population study data and the outliers. In the end, none were used in the final data evaluation to avoid the risk of introducing hidden variables that also correlate with D3.

Mortality rates and D3 blood levels from studies on hospitalized COVID-19 patients were assembled in a separate dataset. When no median D3 blood levels were provided for the individual study cohorts, the IQR, mean±SD or estimated values within the grouping criteria were used in that order. Patient characteristics, including age IQR, sex and diabetes status, were used to compute expected mortality rates with a machine learning model [144] . Based on the expected mortality rate, the observed mortality rates were corrected for the specific cohorts. 

Database and registry searches resulted in 563 and 66 records, respectively. Nonsystematic web searches accounted for 13 studies, from which an additional 31 references were assessed. After removal of 104 duplicates and initial screening, 44 studies remained. Four meta-studies, one comment, one retracted study, one report with unavailable data, one wrong topic report and one Russian language record were excluded. The remaining 35 studies were assessed in full text, 20 of which did not meet the eligibility criteria due to their study design or lack of quantitative mortality data. Four further studies were excluded due to missing data for individual patient cohorts. Finally, three studies were excluded due to skewed or nonrepresentative patient characteristics, as reviewed by LB and JVM [145] [146] [147] . Eight eligible studies for quantitative analysis remained, as listed in Table 2 . A PRISMA flowchart [148] is presented in Figure   3 . Figure 4 . One population study by Ahmad et al. [143] was identified. Therein, the CMRs are compiled for 19

European countries based on COVID-19 pandemic data from Johns Hopkins University [156] Table 3 suggest the sex/age distribution, diabetes and the rigidity of public health measures as some of the causes for outliers within the Ahmad dataset.

However, this has little effect on the further results discussed below. The extracted data from seven hospital studies showed a median vitamin D3 level of 23.2 ng/ml (14.5 -30.9 ng/ml). These data are plotted after correction of patient characteristics and scaling in combination with the data points from Ahmad in Figure 5 . The correlation results are shown in Table 4 in which the combined data show a significant negative Pearson correlation at r(32)=-.3989, p=.0194. The linear regression results can be found in Table 5 . The regression for the combined data intersects the D3 axis at 50.7 ng/ml, suggesting a theoretical point of zero mortality. 

This study illustrates that, at a time when vaccination was not yet available, patients with sufficiently high D3 serum levels preceding the infection were highly unlikely to suffer a fatal outcome. The partial risk at this D3 level seems to vanish under the normal statistical mortality risk for a given age and in light of given comorbidities. This correlation should have been good news when vaccination was not available but instead was widely ignored. Nonetheless, this result may offer hope for combating future variants of the rapidly changing virus as well as the dreaded breakthrough infections, in which severe outcomes have been seen in 10.5% of the vaccinated versus 26.5% of the unvaccinated group [177], with breakthrough even being fatal in 2% of cases [178] .

Could a virus that is spreading so easily and is much deadlier than H1N1 influenza be kept under control if the human immune system could work at its fullest capacity? Zero mortality, a phrase used in the abstract, is of course an impossibility, as there is always a given intrinsic mortality rate for any age. Statistical variations in genetics as well as in lifestyle often prevent us from identifying the exact medical cause of death, especially when risk factors (i.e., comorbidities) and an acute infection are in competition with one another. Risk factors also tend to reinforce each other. In COVID-19, it is common knowledge that type II diabetes, obesity, and high blood pressure easily double the risk of death [160] , depending on age. The discussion of whether a patient has died "because of" or "with" COVID-19 or "from" or only "with" his or her comorbidities thus seems obsolete. SARS-CoV-2 infection is statistically just adding to the overall mortality risk, but obviously to a much higher degree than most other infectious diseases or general risk factors.

The background section has shown that the vitamin D system plays a crucial role not only in the healthiness and strength of the skeletal system (rickets/osteoporosis) but also in the outcome of many infectious and/or autoimmune diseases [161, 162] . Preexisting D3 deficiency is highly correlated in all of these previously mentioned cases.

Many argue that, because a correlation does not imply causality, a low D3 level may be merely a biomarker for an existing disease rather than its cause. However, the range of diseases for which existing empirical evidence shows an inverse relationship between disease severity and long-term D3 levels suggests that this assumption should be reversed [163] .

This study investigated the correlation between vitamin D levels as a marker of a patient's immune defense and resilience against COVID-19 and presumably other respiratory infections. It compared and merged data from two completely different datasets. The strength of the chosen approach lies in its diversity, as data from opposite and independent parts of the data universe yielded similar results. This This is -to our knowledge -the first study that aimed to determine an optimum D3 level to minimize COVID-19 mortality, as other studies typically limit themselves to identifying odds ratios for 2-3 patient cohorts split at 30 ng/ml or lower.

Another study confirmed that the number of infections clearly correlated with the respective D3 levels, with a cohort size close to 200,000 [115] . A minimum number of infections was observed at 55 ng/ml. Does that mean that vitamin D protects people from getting infected? Physically, an infection occurs when viruses or bacteria intercept and enter body cells. Medically, infections are defined as having symptomatic aftereffects. However, a positive PCR test presumes the individual to be infectious even when there are no clinical symptoms and can be followed by quarantine. There is ample evidence that many people with a confirmed SARS-CoV-2 infection have not shown any symptoms [166] .

A "physical infection", which a PCR test can later detect, can only be avoided by physical measures such as disinfection, masks and/or virucidal sprays, which will prevent the virus from either entering the body or otherwise attaching to body cells to infect them. However, if we define "infection" as having to be clinically symptomatic, then we have to refer to it as "silent" to describe what happens when the immune system fights down the virus without showing any symptoms apart from producing specific T-cells or antibodies. Nevertheless, the PCR test will show such people as being "infected/infectious", which justifies that they are counted as "cases" even without confirmation by clinical symptoms, e.g., in

Worldometer Statistics [164] .

Just as the D3 status correlates not only with the severity of symptoms but also with the length of the ongoing disease [165], it is fair to assume that the same reasoning also applies for silent infections. Thus, the duration in which a silent infection itself is active, i.e., infectious and will therefore produce a positive PCR result, may be reduced. We suggest that this may have a clear effect on the reproduction rate.

Thus, it seems clear that a good immune defense, be it naturally present because of good preconditioning or from an acquired cross immunity from earlier human coronavirus infections, cannot "protect" against "infected" patients (confirmed by PCR tests) with a vitamin D level >30 ng/ml [115] does not prove protection against physical infection but rather against its consequences -a reduction in the number of days of people being infectious must statistically lead to the demonstrated result of only half as many positive PCR tests recorded in the group >30 ng/ml vs. the group <30 ng/ml. This "protection" was most effective at ~55 ng/ml, which agrees well with our results.

This result was also confirmed in a 2012 study, which showed that one of the fatal and most feared symptoms of COVID-19, the out-of-control inflammation leading to respiratory failure, is directly correlated with vitamin D levels. Cells incubated in 30 ng/ml vitamin D and above displayed a significantly reduced response to lipopolysaccharides (LPS), with the highest inflammatory inhibition observed at 50 ng/ml [167] .

This result matches scientific data on the natural vitamin D3 levels seen among traditional hunter/gatherer lifestyles in a highly infectious environment, which were 110-125 nmol/L (45-50 ng/ml) [168] .

There is a major discrepancy with the 30 ng/ml D3 value considered by the WHO as the threshold for sufficiency and the 20 ng/ml limit assumed by D-A-CH countries.

Three directors of Iranian Hospital Dubai also state from their practical experience that among 21 COVID-19 patients with D3 levels above 40 ng/ml (supplemented with D3 for up to nine years for ophthalmologic reasons), none remained hospitalized for over 4 days, with no cytokine storm, hypercoagulation or complement deregulation occurring [169] .

Thus, we hypothesize that long-standing supplementation with D3 preceding acute infection will reduce the risk of a fatal outcome to practically nil and generally mitigate the course of the disease. is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this this version posted September 25, 2021. ; https://doi.org/10.1101/2021.09.22.21263977 doi: medRxiv preprint magnesium, zinc, and selenium. Carlberg et al. found this bell curve distribution when verifying the activation of 500 -700 genes contributing to the production of immune system-relevant cells and proteins after D3 supplementation [170] . Participants at the low end showed only 33% activation, while others at the high end showed well over 80% "of the 36 vitamin D3-triggered parameters". Carlberg used the term (vitamin D3) low and high responders to describe what he saw.

This finding may explain why a "D3 deficient" high responder may show only mild or even no symptoms, while a low responder may experience a fatal outcome. It also explains why, on the one hand, many so-called "autoimmune" inflammation-based diseases do highly correlate with the D3 level based on, e.g., higher latitudes or higher age, when D3 production decreases, but why only parts of the population are affected: it is presumably the low responders who are mostly affected. Thus, for 68%-95%

(1 or 2 sigma SDs), the suggested D3 level may be sufficient to fight everyday infections, and for the 2.5%-16% of high responders, it is more than sufficient and is completely harmless. However, for the 2.5%-16% of low responders, this level should be raised further to 75 ng/ml or even >100 ng/ml to achieve the same immune status as mid-level responders. A vitamin D3 test before the start of any supplementation in combination with the patient's personal history of diseases might provide a good indication as to which group the patient belongs to and thus whether 50 ng/ml would be sufficient, or, if "normal" levels of D3 are found (between 20 and 30 ng/ml) along with any of the known D3-dependent autoimmune diseases, a higher level should be targeted as a precaution, especially as levels up to 120 ng/ml are declared to have no adverse effects by the WHO.

As future mutations of the SARS-CoV-2 virus may not be susceptible to the acquired immunity from vaccination or from a preceding infection, the entire population should raise their serum vitamin D level to a safe level as soon as possible. As long as enough vitamin K2 is provided, the suggested D3 levels are entirely safe to achieve by supplementation. However, the body is neither monothematic nor monocausal but a complicated system of dependencies and interactions of many different metabolites, hormones, vitamins, micronutrients, enzymes, etc. Selenium, magnesium, zinc and vitamins A and E should also be A simple observational study could prove or disprove all of the above. If one were to test PCR-positive contacts of an infected person for D3 levels immediately, i.e., before the onset of any symptoms, and then follow them for 4 weeks and relate the course of their symptomatology to the D3 level, the same result as shown above must be obtained: a regression should cross the zero baseline at 45-55 ng/ml. Therefore, we strongly recommend the performance of such a study, which could be carried out with very little human and economic effort.

Even diseases caused by low vitamin D3 levels cannot be entirely resolved by ensuring a certain (fixed) D3 level for the population, as immune system activation varies. However, to fulfill Scribonius Largus' still valid quote "primum non nocere, secundum cavere, tertium sanare" from 50 A.D., it should be the duty of the medical profession to closely look into a medication or supplementation that might help (tertium sanare) as long as it has no known risks (primum non nocere) within the limits of dosages that are needed for the blood level mentioned (secundum cavere).

Unfortunately, this does not imply that in the case of an acute SARS-CoV-2 infection, newly started supplementation with 25(OH)D3 will be a helpful remedy when calcidiol deficiency is evident, especially if this deficiency has been long lasting and caused or exacerbated typical comorbidities that can now aggravate the outcome of the infection. This was not a question we aimed to answer in this study.

This study does not question the vital role that vaccination will play in coping with the COVID-19 pandemic. Nor does it claim that in the case of an acute SARS-CoV-2 infection, a high boost of 25(OH)D3 is or could be a helpful remedy when vitamin D deficiency is evident, as this is another question. Furthermore, empirical data on COVID-19 mortality for vitamin D3 blood levels above 35 ng/ml are sparse. 

Not applicable.

Not applicable.

The datasets generated and/or analyzed during the current study have been made available online [171] .

The authors declare that they have no competing interests.

Not applicable.

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Multiple Sclerosis: Lipids, Lymphocytes, and Vitamin D. Immunometabolism

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Update in vitamin D

Vitamin D: Nutrient, Hormone, and Immunomodulator

Vitamin D: modulator of the immune system

The association between vitamin D deficiency and communityacquired pneumonia

Ovid Technologies

Vitamin D3 and gargling for the prevention of upper respiratory tract infections: a randomized controlled trial

Regulation of Immune Function by Vitamin D and Its Use in Diseases of Immunity

Modulation of the Immune Response to Respiratory Viruses by Vitamin D

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Association of vitamin D deficiency with severity of acute respiratory infection: A case-control study in New Zealand children

The role of vitamin D in prevention and treatment of infection. Inflamm Allergy-Drug Targets (Formerly Curr Drug Targets-Inflammation Allergy)

Translating the role of vitamin D3in infectious diseases

Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data

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Association between prehospital vitamin D status and incident acute respiratory failure in critically ill patients: a retrospective cohort study

D-hormone and the immune system

Can supplementation with vitamin D reduce the risk or modify the course of autoimmune diseases? A systematic review of the literature

Consensus on Vitamin D Deficiency and Insufficiency in Children

Evidence that Vitamin D Supplementation Could Reduce Risk of Influenza and COVID-19 Infections and Deaths

A Statistical Error in the Estimation of the Recommended Dietary Allowance for Vitamin D

The Big Vitamin D Mistake

The Synergistic Interplay between Vitamins D and K for Bone and Cardiovascular Health: A Narrative Review

Proper calcium use: vitamin K2 as a promoter of bone and cardiovascular health

The Dual Role of Vitamin K2 in "Bone-Vascular Crosstalk": Opposite Effects on Bone Loss and Vascular Calcification

Vitamin D receptor stimulation to reduce acute respiratory distress syndrome (ARDS) in patients with coronavirus SARS-CoV-2 infections

Evidence Regarding Vitamin D and Risk of COVID-19 and Its Severity

Lungs as target of COVID-19 infection: Protective common molecular mechanisms of vitamin D and melatonin as a new potential synergistic treatment

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Vitamin D: a negative endocrine regulator of the renin-angiotensin system and blood pressure

Mahdavi AM. A brief review of interplay between vitamin D and angiotensin-converting enzyme 2: Implications for a potential treatment for COVID -19

Vitamin D can prevent COVID-19 infection-induced multiple organ damage

Vitamin D3 as Potential Treatment Adjuncts for COVID-19

Vitamin D supplementation improves cytokine profiles in patients with congestive heart failure: a double-blind, randomized, placebo-controlled trial

Lineage-specific effects of 1, 25-dihydroxyvitamin D3 on the development of effector CD4 T cells

dihydroxyvitamin D3: preferential inhibition of Th1 functions

1$α$, 25-Dihydroxyvitamin D3 has a direct effect on naive CD4+ T cells to enhance the development of Th2 cells

1,25-Dihydroxyvitamin D3 and IL-2 Combine to Inhibit T Cell Production of Inflammatory Cytokines and Promote Development of Regulatory T Cells Expressing CTLA-4 and FoxP3

Evidence for possible association of vitamin D status with cytokine storm and unregulated

The role of cathelicidin and defensins in pulmonary inflammatory diseases

Membrane core-specific antimicrobial action of cathelicidin LL-37 peptide switches between pore and nanofibre formation. Sci Rep

Toll-like receptor triggering of a vitamin D-mediated human antimicrobial response. Science (80-)

Human cathelicidin antimicrobial peptide (CAMP) gene is a direct target of the vitamin D receptor and is strongly up-regulated in myeloid cells by 1,25-dihydroxyvitamin D 3

Vitamin D and the anti-viral state

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dysfunction: Implications for patients with COVID-19

Vitamin D and Endothelial Function

Role of vitamin D in treating COVID-19-associated coagulopathy: problems and perspectives

Emerging role of vitamin D and its associated molecules in pathways related to pathogenesis of thrombosis

Coagulation abnormalities and thrombosis in patients with COVID-19

Possible association of vitamin D status with lung involvement and outcome in patients with COVID-19: a retrospective study

Serum 25(OH)D Level on Hospital Admission Associated With COVID-19 Stage and Mortality

Serum Vitamin D levels are associated with increased COVID-19 severity and mortality independent of visceral adiposity. Cold Spring Harbor Laboratory

Vitamin D deficiency as a predictor of poor prognosis in patients with acute respiratory failure due to COVID-19

Impact of Vitamin D Deficiency on COVID-19 --A Prospective Analysis from the CovILD Registry

Vitamin D insufficiency is prevalent in severe COVID-19. Cold Spring Harbor Laboratory

Incidence and Disease Severity in Chinese People

Vitamin D deficiency correlates with a reduced number of natural killer cells in intensive care unit (ICU) and non-ICU patients with COVID-19 pneumonia

Association of Vitamin D Status and Other Clinical Characteristics With COVID-19 Test Results

SARS-CoV-2 positivity rates associated with circulating 25-hydroxyvitamin D levels

Low plasma 25(OH) vitamin D level is associated with increased risk of COVID-19 infection: an Israeli population-based study

Vitamin D sufficiency, a serum 25-hydroxyvitamin D at least 30 ng/mL reduced risk for adverse clinical outcomes in patients with COVID-19 infection

Vitamin D Deficiency and Outcome of COVID-19 Patients

An Evaluation of Serum 25-Hydroxy Vitamin D Levels in Patients with COVID-19 in New York City

Vitamin D Status and COVID-19 Clinical Outcomes in Hospitalized Patients

Vitamin D supplementation in COVID-19 patients: a clinical case series

Short term, high-dose vitamin D supplementation for COVID-19 disease: a randomised, placebo-controlled, study (SHADE study)

Vitamin D Supplementation Associated to Better Survival in Hospitalized Frail Elderly COVID-19 Patients: The GERIA-COVID Quasi-Experimental Study

High dose vitamin D administration in ventilated intensive care unit patients: A

Low serum 25-hydroxyvitamin D (25[OH]D) levels in patients hospitalized with COVID-19 are associated with greater disease severity

Effect of calcifediol treatment and best available therapy versus best available therapy on intensive care unit admission and mortality among patients hospitalized for COVID-19: A pilot randomized clinical study{\ textquotedblright}

Vitamin D and survival in COVID-19 patients: A quasi-experimental study

Vitamin D deficiency and the COVID-19 pandemic

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A Basic Review of the Preliminary Evidence That COVID-19 Risk and Severity Is Increased in Vitamin D Deficiency. Front Public Heal

Does serum 25-hydroxyvitamin D decrease during acute-phase response? A systematic review

Point of view: Should COVID-19 patients be supplemented with vitamin D? Maturitas

Mean Vitamin D levels in 19 European Countries & COVID-19 Mortality over 10 months. Cold Spring Harbor Laboratory

An interactive web-based dashboard to track COVID-19 in real time

Response2covid19, a dataset of governments' responses to COVID-19 all around the world. Sci Data

The role of vitamin D in the prevention of coronavirus disease 2019 infection and mortality

Vitamin D supplementation and clinical outcomes in COVID-19: a systematic review and meta-analysis

The impact of vitamin D supplementation on mortality rate and clinical outcomes of COVID-19 patients: A systematic review and meta-analysis. Cold Spring Harbor Laboratory

Vitamin D supplementation and Covid-19 outcomes: A systematic review, meta-analysis and meta-regression

European Countries & COVID-19 Mortality over 10 months

Circulating Vitamin D levels status and clinical prognostic indices in COVID-19 patients

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Vitamin D Deficiency and Low Serum Calcium as Predictors of Poor Prognosis in Patients with Severe COVID-19

The PRISMA 2020 statement: An updated guideline for reporting systematic reviews

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Low Levels of Vitamin D were Associated with Coagulopathy Among Hospitalized Coronavirus Disease-19 (COVID-19) Patients: A Single-Centered Study in Indonesia

COVID-19 Clinical Outcomes in Hospitalized Patients

Serum Vitamin D levels are associated with increased COVID-19 severity and mortality independent of visceral adiposity

Vitamin D Status is Associated With In-hospital Mortality and Mechanical Ventilation: A Cohort of COVID-19 Hospitalized Patients

Association of Vitamin D Status With Hospital Morbidity and Mortality in Adult Hospitalized COVID-19 Patients With COVID-19

An Evaluation of Serum 25-Hydroxy Vitamin D Levels in Patients with COVID-19 in New York City

An interactive web-based dashboard to track COVID-19 in real time

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Population interpolated by single age and single year

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Emerging role of vitamin D in 40 the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) preprint The copyright holder for this this version posted

The Implication of Vitamin D and Autoimmunity: a Comprehensive Review

Sugar consumption, metabolic disease and obesity: The state of the controversy

Influence of vitamin D status on hospital length of stay and prognosis in hospitalized patients with moderate to severe COVID-19: a multicenter prospective cohort study

Robust T Cell Immunity in Convalescent Individuals with Asymptomatic or Mild COVID-19

Vitamin D inhibits monocyte/macrophage proinflammatory cytokine production by targeting MAPK phosphatase-1

Traditionally living populations in East Africa have a mean serum 25-hydroxyvitamin D concentration of 115 nmol/l

Suggested role of Vitamin D supplementation in COVID-19 severity

Relevance of vitamin D receptor target genes for monitoring the vitamin D responsiveness of primary human cells

COVID-19 mortality risk correlates inversely with vitamin D3 status, mortality close to zero could theoretically be achieved at 50 ng/ml 25(OH)D3: Results of a systematic 41 the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) preprint The copyright holder for this this version posted

Vitamin D and COVID-19: Lessons from Spaceflight Analogs

Vitamin D Double-edged Sword Against COVID-19

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Aldosterone System and Vitamin D-FGF-23-klotho in Chronic Kidney Disease: Figure 1

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Because it is a Negative Acute Phase Reactant

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Infection: Systematic Review and Meta-Analysis. Front Public Heal

It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) preprintThe copyright holder for this this version posted September 25, 2021. ; https://doi.org/10.1101/2021.09.22.21263977 doi: medRxiv preprint It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) preprint