During winter, the sun's emission of ultraviolet B rays decreases.
The sun emits the same amount of UV B rays. The atmosphere filters UV B. Since the sunlight hits the earth at an angle, it filters more UV B rays in the winter.
So, here is the full scoop: UV B rays are dangerous. They cause cancer.
UV B rays also happen to convert cholesterol in the skin into Vitamin D3. The kidneys convert it into vitamin D2. Vitamin D regulates bone growth and the immune system.
There appears to be a correlation between vitamin D deficiency and the severity of COVID19.
The challenge is to get enough exposure to the skin to produce vitamin D, but not enough to get skin cancer.
The skin of people in northern climates turn white in the winter in an effort to get more vitamin D and tan in the summer to reduce risk of skin cancer.
The best time to catch UV B rays is at noon. But I think that people should limit their exposure.