More than two dozen studies of Covid-19 show us the way to TURN BACK the Second Wave.
Vitamin D versus Covid-19
These 27 studies show that having normal blood levels of vitamin D reduces Covid-19 risks:
* vitamin D reduces risk of infection [7, 8, 9, 11, 12, 14, 16, 23, 27]
* vitamin D reduces risk of having a severe case [1, 3, 4, 5, 15, 17, 20, 22, 24, 25, 26]
* vitamin D reduces risk of hospitalization, ICU care, or mechanical ventilation [2, 10, 14, 15, 21, 22, 24, 26]
* vitamin D reduces risk of dying from Covid-19 [4, 6, 7, 9, 12, 13, 17, 18, 19, 22, 24, 25]
And all you need to get those benefits is a normal blood level of vitamin D:
30 to 100 ng/ml, which is the same as 75 to 250 nmol/liter.
Note, however, that one study  found that vitamin D levels in the range of 50 to 60 ng/ml had the lowest risk of infection from Covid-19, about half the risk of the blood level 20 ng/ml. But the other studies show that 30 ng/ml (or higher) is enough.
The type of vitamin D measured by lab tests in the blood is called “25(OH)D”.
“Vitamin D deficiency is defined as a 25(OH)D below 20 ng/ml (50 nmol/liter), and vitamin D insufficiency as a 25(OH)D of 21–29 ng/ml (525–725 nmol/liter).” 
What is causing the Second Wave? Vitamin D blood levels are seasonal; they rise and fall from one season to another. In summer, vitamin D levels are higher because people are out in the sunshine. When sunshine (specifically UV-B) strikes the skin, the body makes vitamin D. But as people spend more time indoors, in autumn and winter, vitamin D blood levels fall. The levels decrease from late September to October to November, and they reach their lowest extent in December through March.
This type of seasonal vitamin D deficiency and insufficiency is VERY COMMON in nations of the northern hemisphere, especially further north. The Southern States in the U.S. have this problem, but to a lesser extent than the States at higher latitudes in the U.S. Then Europe and Canada also have low vitamin D in autumn and winter.
In Muslim nations and in other nations where the religion or social custom requires covering the skin so that little skin is exposed to sunshine, vitamin D deficiency can be quite widespread. It is too difficult to obtain vitamin D from food, unless it is fortified with vitamin D. But even when, it would be too difficult to get the right amount of vitamin D to everyone in the right dosage through food. Perhaps religious or social rules could be loosened by authorities, so as to permit “sunshine on skin” to make vitamin D. The other possibility is vitamin D supplementation (discussed further below).
In the United States, where the Second Wave will strike next starting in October, more than twice as many adults have a vitamin D deficiency in winter (48%) than in summer (21%) . That increases the number of persons infected with Covid-19, which greatly increases the spread (as the disease is highly contagious), and it also increases the severity of those cases that have low vitamin D, resulting in a much greater need for hospital beds, ICU beds, and ventilators. Low vitamin D in winter increases the fatality rate (percent who die) on top of a higher case rate, meaning that the number of deaths rises faster than the number of cases, since the cases are more severe […]