Does vitamin D deficiency increase COVID-19 risk?

A new paper published on the preprint server medRxiv* in May 2020 reports the first evidence that vitamin D deficiency and inadequate treatment could increase the chances of testing positive for COVID-19. This could shape the way for screening individuals at risk for vitamin D deficiency.

Classifying Vitamin D Deficiency

The study by researchers at the University of Chicago looked at over 4,300 patients with COVID-19 of whom 499 had been tested for vitamin D levels in the previous year. Based on this, they were divided into likely deficient (25%), likely adequate (58%), and uncertain (16%).

Vitamin D deficiency was diagnosed by “the most recent 25-hydroxycholecalciferol <20ng/ml or 1,25-dihydroxycholecalciferol <18pg/ml within one year before COVID-19 testing.”

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources

Vitamin D Deficiency Linked to Positive COVID-19 Test

Multiple variables were analyzed in this study. The researchers found that people with vitamin D deficiency at the last testing, and who did not receive higher doses of vitamin D (keeping them deficient in all probability), are much more likely to be infected with the virus than those with probably sufficient levels. The corresponding rates of infection were 22% vs. 12%.

The prevalence of vitamin D deficiency decreased with higher vitamin D dosage, but the latter was not linked to positive testing for COVID-19.

Earlier trials and meta-analyses have already suggested that sufficient vitamin D can prevent COVID-19. Still, this study is the earliest direct connection between vitamin D deficiency and a higher risk of viral illness. Vitamin D treatment can reduce the risk of other respiratory infections as well, with coronaviruses playing a leading role in them.

The analysis failed to throw up a significant difference between patients with low vitamin D levels at last testing who were or were not treated. On the other hand, the study shows that few such patients actually receive high doses of vitamin D. In other words, the reluctance to treat this deficiency adequately is a significant contributor to the failure to detect an association.

Factors Underlying Vitamin D Deficiency

Vitamin D deficiency is more common in black Americans due to the dense concentration of the ultraviolet-blocking chemical in the skin. Since this community has been disproportionately affected by COVID-19, in terms of both sickness and death, it would be wise to offer vitamin D testing to them.

Genetic factors affect the incidence of vitamin D deficiency. This finding may indicate a heritable component of the risk for the viral illness. This could help determine whether to test for COVID-19 among relatives of people who have the infection.

Another question that is raised by this study is whether vitamin D supplementation could reduce the transmission of the virus. Vitamin D is an immunomodulator, acting through the innate immune cells called dendritic cells as well as the adaptive T cells. This leads to higher clearance of the virus from the body as well as suppression of the inflammatory responses which lead to symptomatic illness.

A higher level of vitamin D is linked to a reduction in IL-6 levels, the latter being a proinflammatory cytokine. This is a significant discovery since IL-6 inhibitors are a major drug target to suppress the cytokine storm that is thought to be linked to death and critical illness.

The vitamin may also influence the processing of zinc, which reduces coronavirus replication. All the above actions are beneficial and can help to control viral spread.

However, the anti-inflammatory or immunomodulatory activity of vitamin D could also increase the number of asymptomatic infections, and thus possibly increase the number of people infected by secondary contact. The absence of cough, fever, and other expected manifestations could confuse the issue by increasing the asymptomatic patient pool. Since this has the potential to increase community spread, it is difficult to gauge the degree of protection in individuals without deficiency.

Limitations and Implications

The study has its drawbacks. The observational nature of the study does not allow the conclusion that vitamin D deficiency causes an increased risk of COVID-19 infection. Indeed, vitamin D may be reduced because of multiple medical conditions and behavioral factors that quite possibly reduce the chance of adequate treatment while boosting the COVID-19 risk.

However, the findings remain unchanged over a wide range of other disease conditions and age groups.

The study also examined patients who had received vitamin D2 or calcitriol. The use of calcitriol was one of the factors that led to a definition of vitamin D deficiency since it is often used in patients with chronic renal disease or hypoparathyroidism. When these cases are excluded, the observed associations typically become stronger.

Another factor is the study location and base: a large number of people with chronic illness, health workers, and Black Americans, in a northern city in winter. This predisposes to a more significant number of people with vitamin D deficiency. In another population subgroup, this may be less important in determining COVID-19 risk.

The Case for Universal Vitamin D Supplementation

The findings are in support of the finding that a low vitamin D level boosts the risk of COVID-19, but treatment that adequately addresses the deficiency reduces the risk. This conclusion is strengthened by the finding that those who were treated with higher doses after having a low vitamin D level at their last test were not at higher risk. Their chances were comparable to those whose last test results and treatment history suggested adequate vitamin D levels at the time of COVID-19 testing.

The type and dosage of treatment are related to the rate of vitamin D deficiency, and this can be explained entirely by the administration of vitamin D3 at 2000 IU of ≥3000IU. This relationship shows that treatment is an important factor in determining this risk.

On the other hand, the relationship between treatments for vitamin D deficiency and the risk for COVID-19, and to the level of vitamin D and its deficiency, is intricate. It underlines the need to carry out randomized trials that can settle the matter with more significant evidence for or against the role of treatment of vitamin D deficiency in reducing this risk.

The importance of social distancing and restriction of movement in COVID-19 makes inexpensive home tests for vitamin D even more valuable against the background of its contribution to deciding the type of treatment. On the other hand, vitamin D may be blindly given at 4000IU or 5000IU a day because of its known safety, as long as the person is not known to have any contraindication to this supplement,.

Logically, the preventive effect of vitamin D treatment would be most significant among those who have low levels of the sunshine vitamin. Despite this expectation, the researchers argue for supplementation to be available to all, and if required, for testing directed at high-risk groups that are likely to have deficient levels of Vitamin D and/or for COVID-19.

This is the more feasible considering that vitamin D supplements are quite safe, COVID-19 tests are limited even today, tests are expensive and vitamin D levels fluctuate with the seasons due to variable exposure to the strong ultraviolet rays needed to convert the prodrug to its active form.

The type of analysis performed here is quite simple and lends itself to other situations, with every likelihood of achieving similar conclusions. This is especially so because vitamin D deficiency is so prevalent in the American population, and particularly in darker-skinned people, older individuals, and those living in nursing homes.

Given these findings, the researchers recommend “expanded population-level vitamin D treatment and testing and assessment of the effects of those interventions,” because of the strong possibility that vitamin D deficiency increases COVID-19 risk. This should, therefore, be followed up with more studies at once to understand whether positive measures to increase vitamin D levels in deficient people would help reduce the number of new COVID-19 cases. Large-scale interventions aimed at the people at large, as well as high-risk group-specific interventions, should be prioritized.

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources

Journal references:

Article Revisions

  • Mar 20 2023 - The preprint preliminary research paper that this article was based upon was accepted for publication in a peer-reviewed Scientific Journal. This article was edited accordingly to include a link to the final peer-reviewed paper, now shown in the sources section.
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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  1. John Lambiase John Lambiase United States says:

    I am a Clinical Lab Scientist and I have been data mining Vitamin D in acute respiratory infections and it's potential link to Sars Cov 2 Virus since ex CDC director Tom Freiden went on the news on March 23rd.  With that being said, this study in the article out of Chicago was not the first study linking Vitamin D deficiency to Covid 19.  I believe that designation with formidable proof goes to Dr. Mark Alipio from Indonesia.

    Alipio, Mark, Vitamin D Supplementation Could Possibly Improve Clinical Outcomes of Patients Infected with Coronavirus-2019 (COVID-19) (April 9, 2020). Available at SSRN: or

    With that being said, there is seemingly large amounts of pushback from the medical community and it is a little disturbing.  Yes I get it, the Medical societies really push real evidence based medicine, but there has to be a give and take given the safety for people to take 3000 iu of Vitamin D vs the potential risk of possibly getting severe Covid 19 manifestation should be a no brainer.  I mean good grief I do not recall anything every having this much and magnitude of circumstancial or anecdotal evidence and every day it becomes more and more.  Until the studies are done and complete what is the possible harm?  So we are willing to wait for direct evidence while houndreds of thousands die?  Ok, so the new oath in Medicine has transcended to "First do no harm" to "First do Nothing"

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
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