Coronavirus disease 2019 (COVID-19) is a contagious disease caused by a virus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The first known case was identified in Wuhan, China, in December 2019.[7] The disease spread worldwide, leading to the COVID-19 pandemic.
Symptoms of COVID‑19 are variable, but often include fever,[8] cough, headache,[9] fatigue, breathing difficulties, loss of smell, and loss of taste.[10][11][12] Symptoms may begin one to fourteen days after exposure to the virus. At least a third of people who are infected do not develop noticeable symptoms.[13] Of those people who develop symptoms noticeable enough to be classed as patients, most (81%) develop mild to moderate symptoms (up to mild pneumonia), while 14% develop severe symptoms (dyspnea, hypoxia, or more than 50% lung involvement on imaging), and 5% develop critical symptoms (respiratory failure, shock, or multiorgan dysfunction).[14] Older people are at a higher risk of developing severe symptoms. Some people continue to experience a range of effects (long COVID) for months after recovery, and damage to organs has been observed.[15] Multi-year studies are underway to further investigate the long-term effects of the disease.[15]
COVID‑19 transmits when people breathe in air contaminated by droplets and small airborne particles containing the virus. The risk of breathing these in is highest when people are in close proximity, but they can be inhaled over longer distances, particularly indoors. Transmission can also occur if splashed or sprayed with contaminated fluids in the eyes, nose or mouth, and, rarely, via contaminated surfaces. People remain contagious for up to 20 days, and can spread the virus even if they do not develop symptoms.[16][17] Vitamin D is a group of fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and many other biological effects.[1][2][3] In humans, the most important compounds in this group are vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol).[2][3][4]
The major natural source of the vitamin is synthesis of cholecalciferol in the lower layers of epidermis of the skin through a chemical reaction that is dependent on sun exposure (specifically UVB radiation).[1][5] Cholecalciferol and ergocalciferol can be ingested from the diet and supplements.[1][2] Only a few foods, such as the flesh of fatty fish, naturally contain significant amounts of vitamin D.[2][6] In the U.S. and other countries, cow’s milk and plant-derived milk substitutes are fortified with vitamin D, as are many breakfast cereals.[1] Mushrooms exposed to ultraviolet light contribute useful amounts of vitamin D2.[2] Dietary recommendations typically assume that all of a person’s vitamin D is taken by mouth, because sun exposure in the population is variable and recommendations about the amount of sun exposure that is safe are uncertain in view of the skin cancer risk.[2]
Vitamin D from the diet, or from skin synthesis, is biologically inactive. It is activated by two protein enzyme hydroxylation steps, the first in the liver and the second in the kidneys.[1][4] Because vitamin D can be synthesized in adequate amounts by most mammals if they get enough sunlight, it is not essential and therefore is technically not a vitamin.[3] Instead it can be considered a hormone, with activation of the vitamin D pro-hormone resulting in the active form, calcitriol, which then produces effects via a nuclear receptor in multiple locations.[3]
