Despite being heavily shared on social media, this study does not demonstrate a relationship between Vitamin D deficiency and the severity of COVID infection.
- There was no statistically significant difference in the level of vitamin D in the blood of patients with more severe cases of COVID-19
- This study design could not prove that Vitamin D leads to more severe COVID infection
- Although there is a higher prevalence of Vitamin D deficiency in African Americans and they have had a disproportionate number of deaths due to COVID-19, there are many other factors such as unequal access to medical care and health disparities that could explain this finding.
Why is this important?
Vitamin D supplementation is relatively cheap and safe. If low vitamin D levels were related to COVID-19 disease severity, then supplementing with Vitamin D pills would be an easy fix.
They chose to study the relationship between Vitamin D and COVID-19 severity because there is evidence in animals and in cells that Vitamin D is important for regulating the immune system in the lungs. There is some evidence that people with low Vitamin D levels get more lung infections in general. But, having higher Vitamin D levels might just be a marker for overall healthiness, eating better, getting more sunlight, or many other confounding factors unrelated to lung health. From the data, it’s not clear if we should be giving people Vitamin D supplements to protect their lungs. There is some evidence that giving vitamin D prior to some types of surgery 1 or to people with low vitamin D levels can protect the lungs 2; there is also a large study showing that giving a large dose of vitamin D to critically ill people does not improve their chance of survival 3.
What did the study do?
Collected hospital records from COVID-19 patients
The researchers looked at the records of patients at their hospital and found 20 patients with a diagnosis of COVID-19 and vitamin D levels that were measured during their hospitalization.
Unresolved chicken and the egg problem
This study didn’t look at the vitamin D levels before the patients got sick. Other papers have shown that Vitamin D levels decrease with infectious illness. The relationship may be that more severe COVID infection causes low vitamin D, rather than low vitamin D causes more severe COVID infection.
Since this was a retrospective study (looking at existing charts), it's impossible to make conclusions about causation.
Examined relation between Vitamin D and COVID-19
No statistical significance
The authors did not find a statistically significant association between Vitamin D deficiency and severity of COVID-19 infection.
Not enough patients to draw meaningful conclusions
The study was limited to 20 patients, 13 of which were in the ICU and 7 which were treated on the hospital floor. This is an extremely small number of patients for this type of research study.
Did not compare to an appropriate control group
Their choice of control group is questionable in their statistical analysis. They compared vitamin D levels of their patients to vitamin D levels of all patients in the ICU, as well as comparing the vitamin D levels of COVID patients who required intensive treatment compared to the patients who did not. A more relevant analysis would be to compare the COVID patients with other ICU patients who are there for respiratory illnesses.
Vitamin D deficiency is linked with severe COVID-19
Drawing incorrect or incomplete conclusions from previous studies
There are several examples of papers they cited which do not support their assertions. For instance the researchers assert that homeless people have a near 100% rate of asymptomatic cases of COVID based on a paper by Baggett et al. 4 The cited paper, however, discusses how the original 15 patients who were diagnosed with COVID at the homeless shelter were removed after developing symptoms, and before the study took place. Then the researchers performed PCR tests, showing active viral infections in ~36% of the population of the homeless shelter, however these patients were never followed up with after the initial test, to see if any developed symptoms or needed hospitalization. The conclusion of the cited paper was simply that the rate of infection at homeless shelters, even following CDC guidelines, is staggeringly high.
The researchers also tried to connect the severity of COVID outbreak in several countries to those countries’ known levels of vitamin D insufficiency. They used Spain, Italy, and Greece as countries with a known high level of vitamin D insufficiency 5 and a severe COVID outbreak, and contrasted that against Scandanavian countries, which are known to have a low level of vitamin D insufficiency due to their diets. They neglected to mention that one of the countries with a very low level of Vitamin D insufficiency is Belgium, which has the highest rate of COVID fatality in the world (when considering countries with +1M population). Also, Greece has a fairly low level of COVID mortality, even though it has one of the highest prevalences of vitamin D insufficiency. The researchers never performed any statistical analysis in order to connect a country’s severity of COVID outbreak to prevalence of vitamin D insufficiency, and only used a few anecdotal cases as evidence.
Claims not supported by evidence or data
One of the assertions in the paper is that homeless people have a low prevalence of vitamin D insufficiency. The researchers, however, never put forth any evidence to support this claim. They simply base this on the fact that homeless people spend a large portion of their time outside in the sun, and that the sun is one of the main sources of vitamin D generation in the body.
Ignoring confounding factors
The authors argue that COVID-19 affects the African American population more than the rest of society and that this may be due to a higher prevalence of vitamin D insufficiency. While this is possible, black patients also have many other health disparities compared to other ethnic groups in the US. There are many factors as to why African Americans suffer disproportionately from COVID-19.
This study did not find a statistically significant link between low Vitamin D and ending up in the ICU. The author’s conclusion is not supported by their own analysis.