The Link Between Vitamin D Deficiency and COVID-19 Mortality


Vitamin D has been previously identified as inhibiting pro-inflammatory cytokines in respiratory tract infections.

Two recent clinical trials have been recently published, reaffirming prior preliminary studies which have asserted that vitamin D deficiency is significantly associated with severe health complications, including death, in patients infected with the novel coronavirus (COVID-19).
Research prior to the COVID-19 pandemic indicates that healthy vitamin D levels may help to mitigate the susceptibility to and severity of a number of acute respiratory tract infections. Vitamin D is known to modulate the response of white blood cells, preventing them from releasing too many inflammatory cytokines.
Vitamin D levels across populations are often highly disparate due to the nature in which it is absorbed through sunlight. In northern countries, where populations are exposed to less sunlight, vitamin D deficiency is often more prevalent. Skin pigmentation also plays a role in how well vitamin D is synthesized through sunlight absorption- people with higher melanin content in their skin, giving it a darker appearance, are often disproportionately affected by vitamin D deficiency.
Researchers at Michigan University have hypothesized that disparities in vitamin D levels may be a compounding factor in the disproportionate death rates among black Americans around the country. In Michigan, over 40% of COVID-19 deaths are African Americans, while African Americans make up only 14% of the population. These findings are just a compounding factor among persistent inequities along racial lines that exist within the health outcomes of U.S. citizens, however, according to Michigan Medicine’s David Brown, MD. Black Americans face higher rates of hypertension, diabetes, and obesity, all of which have been identified as risk factors from COVID-19. Because of ubiquitous healthcare inequities, and other material stressors faced by black Americans at a disproportionate rate, researchers said, the role that vitamin D has in addressing racial disparities may be prone to being outsized in attempts to understand which health factors are most responsible for the disparity.  
Among countries with high COVID-19 mortality rates are Italy and Spain, which researchers have noted are countries in which the elderly are known to avoid strong sunlight. Additionally, the highest levels of vitamin D are found in Scandinavian countries, due to high consumption of cod liver oil and vitamin D supplements. Scandinavian countries also have the lowest number of COVID-19 cases and mortality rates per head of population throughout Europe.
Northwestern University Study
One study, led by Northwestern University, involved a statistical analysis of data from hospitals and clinics across China, France, Germany, Italy, Iran, South Korea, Spain, Switzerland, the U.K., and the U.S. Patients from countries with higher COVID-19 mortality rates such as Italy, Spain, and the U.K., had lower levels of vitamin D compared to patients in countries that were not as severely affected.
According to researchers, these results do not conclusively affirm that everyone should begin “hoarding supplements.”
“While I think it is important for people to know that vitamin D deficiency might play a role in mortality, we don’t need to push vitamin D on everybody,” Northwestern’s Vadim Backman, who led the study, said. “This needs further study, and I hope our work will stimulate interest in this area. The data also may illuminate the mechanism of mortality, which, if proven, could lead to new therapeutic targets.”
Backman said that he harbored skepticism toward hypotheses that things differences in healthcare quality, age distributions in population, testing rates, or different strains of the coronavirus might be responsible for the disparities in mortality rates.
“None of these factors appeared to play a significant role,” Backman said. “The healthcare system in northern Italy is one of the best in the world. Differences in mortality exist even if one looks across the same age group. And, while the restrictions on testing do indeed vary, the disparities in mortality still exist even when we looked at countries or populations for which similar testing rates apply.”
Backman said that the strongest correlation he and his team found was with vitamin D deficiency, using publicly available data from around the globe on median vitamin D levels and, in COVID-19 patients, the rates of both mortality and cytokine storm (a hyperinflammatory condition caused by an overactive immune system). Cytokine storm is a condition which has been attributed to severe lung damage and acute respiratory distress symptoms in patients who have either died or suffered the worst health outcomes.
Backman said that he believes that, if vitamin D levels worldwide were optimal, it could “be as high as cutting the mortality rate in half. It will not prevent a patient from contracting the virus, but it may reduce complications and prevent death in those who are infected.”
Backman cautioned that little is known about what dose of vitamin D would be optimal for settings in which people might be exposed to COVID-19; while vitamin D deficiency is harmful, an excessive intake of vitamin D is also associated with negative side effects.
Anglia Ruskin University Study
A separate study, led by Dr. Lee Smith of Anglia Ruskin University and published in the journal Aging Clinical and Experimental Research, evaluated the association between low average levels of vitamin D and high numbers of COVID-19 cases and mortality rates across 20 European countries.
“Vitamin D has been shown to protect against acute respiratory infections, and older adults, the group most deficient in vitamin D, are also the ones most seriously affected by COVID-19,” Smith said. “A previous study found that 75% of people in institutions, such as hospitals and care homes, were severely deficient in vitamin D. We suggest it would be advisable to perform dedicated studies looking at vitamin D levels in COVID-19 patients with different degrees of disease severity.”