It was a simple study design. Retrospective and observational, in today's science, a fishing trip for correlation by computer. All the patients with a positive COVID-19 test during March through June of this year from Quest served as the population, which was refined to include only those who had a Vitamin D level drawn in the previous year. A patient's ethnicity was "inferred" from their zip code based upon the 2018 America Community Study, a government study done every five years that reports on the many demographics of our country. 88% of those with positive COVID-19 tests also had a Vitamin D test, so the study group consisted of 192,000 individuals from across the nation.

"There was an association between lower SARS-CoV-2 positivity rates and higher circulating 25(OH)D levels."

The researchers acknowledge the limitation that their population was selected based on how symptomatic they might have been; after all, the request for COVID-19 testing was prompted by some concern on the part of the patient or physician. They also note that inferring ethnicity from zip codes may be problematic. Is this a sufficient mea culpa?

Not really. There is a second selection bias left unstated and more problematic why were these individuals having Vitamin D levels determined? It is not a routine laboratory test. There are a variety of conditions that may result in a deficiency. There are several that have been noted in other settings to be COVID-19 risk factors. They include obesity and its surgical management, chronic kidney disease, immobility that would leave you housebound or in a nursing facility, a higher risk of fractures, and skin color; darker-skinned individuals are less able to produce Vitamin D given similar exposure to the sun. Other recognized causes of Vitamin D deficiency include medications used to treat HIV, epilepsy, and elevated cholesterol. I suspect because the authors do not elucidate that the population under study were those who had a suspected deficiency in Vitamin D, who coincidentally had COVID-19 tests. That changes the denominator and makes the sample size of their population much different.

There are a lot of unanswered questions about Vitamin D. We know that it is essential, but what constitutes deficiency remains a product of consensus. Values in the group that the researchers identified as deficient are in that consensus. But the use of italics in categorizing that level as a "deficiency" is the nuanced signal that the word choice itself is a bit biased. Because physicians have no hard and fast cut-off, estimates of Vitamin D deficiency in the population range from 50% to 100%. The lay press has identified many early symptoms of a possible Vitamin D deficiency, including fatigue, frequent colds, bone, muscle and back pain, hair loss, and depression. When coupled with an inexpensive, non-prescription treatment, you have the setting for screening more low risk than high-risk patients. The CDCreportsthat Vitamin D testing increased 83-fold between 2000 and 2010 among Medicare beneficiaries, an admittedly higher risk group.

This study sheds no additional light or insights. It merely adds to the growing literature that Vitamin D may or may not be involved in, whatever disease you wish to mention.

Source: SARS-CoV-2 positivity rates associated with circulating 25-hydroxyvitamin D levels PLOS DOI: 10.1371/journal.pone.0239252

More:
Can Vitamin D Play a Role in Treating or Preventing COVID-19? - American Council on Science and Health

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