Vitamin D deficiency increases risk for becoming infected with COVID-19, and for complications and death from the disease.
• A study of 500 patients showed that those who had low blood levels of hydroxy vitamin D (<20ng/mL) were twice as likely as those with normal levels to develop COVID-19 (JAMA Netw Open, Sept 3, 2020;3(9):e2019722). Hydroxy vitamin D is measured instead of vitamin D itself because vitamin D levels do not drop until after you are already severely depleted of that vitamin.
• Vitamin D can reduce the growth of SARS-CoV-2 (the virus that causes COVID-19) in tissue culture (Int J Mol Sci, 2019;20:1–17).
• COVID-19 can kill people by causing an overactive immune system called “cytokine storm.” Vitamin D helps to reduce cytokine storm by decreasing the production of T helper cells and inflammatory cytokines (APMI, 2019;127:681–687), and by increasing anti-inflammatory cytokines (Nutrients, 2020;12:E236).
• Vitamin D deficiency is associated with comorbidities of COVID-19, such as obesity, diabetes, older age or darker skin (NFS Journal, Aug 2020;20:10–21; BMJ Nutrition, Prevention & Health, May 13, 2020).
• Low vitamin D status is worsened by isolation indoors and lack of sunlight.
• Several studies that have not yet been peer reviewed suggest that vitamin D helps people to fight COVID-19 (medRxiv, June 22, 2020), and that vitamin D deficiency has been associated with increased death rate from COVID-19 in European countries (European Journal of Endocrinology, medRxiv, May 7, 2020). However, there is no evidence that high doses of vitamin D offer greater protection from COVID-19 than moderate doses.

How Common is Vitamin D Deficiency?
About 42 percent of North Americans have low levels of vitamin D, including 82 percent of African-Americans and 70 percent of Hispanics (Nutr Res, Jan 2011;31(1):48-54). You need about 800 IU of vitamin D per day from sunlight exposure, the foods that you eat, or vitamin D pills. Most North Americans meet their needs for vitamin D in the summer, but many need supplements or fortified foods in the winter months.

How Much Vitamin D Do You Need?
The dose of vitamin D recommended by the National Academy of Sciences is 600 international units daily for those up to 70 years of age, and 800 IU for those over 71. People with lighter skin can get the vitamin D they need from about 10 minutes of exposure to sunlight three or four days a week, but the darker your skin, the more time or skin area of exposure you will need. Vitamin D-fortified foods include milk, yogurt, soy beverages, orange juice and cereals; check the nutrition information labels. Fatty fish such as salmon is also a good source of vitamin D. People who are vitamin D deficient (under 20 ng/mL) may need to take in 1000 to 2000 IU/day to get their hydroxy vitamin D levels to normal (Ann Intern Med, Mar 20, 2012;156(6):425-37). Obese people may need even more.

How Much is Too Much?
Doses of vitamin D greater than 4,000 IUs can increase risk for kidney stones, calcification of blood vessels, low parathyroid hormone levels, and even heart attacks. See Large Doses of Vitamin D Can Be Harmful. It is true that the sun’s rays convert cholesterol in the skin to vitamin D, but it is not possible for sunlight to raise vitamin D levels too high, because the sun’s rays destroy any vitamin D in the skin.

My Recommendations
Since vitamin D deficiency appears to increase risk for infection with COVID-19, and for dying from complications of the disease, I recommend getting a blood test for hydroxy vitamin D. If your blood levels of hydroxy vitamin D are below 20 ng/mL, take vitamin D3 pills of at least 1000 IU/day for one to two months. If your blood levels of vitamin D do not return to normal (>20 ng/mL), check with your doctor about taking higher doses or getting a medical evaluation for the cause of your deficiency. See Sunlight: More Than Vitamin D