Acute viral respiratory infections, including COVID-19, are strongly correlated with vitamin D insufficiency. They are also strongly seasonal, peaking in the winter, when the availability of the UVB component of sunlight is low. UVB is known for inducing production of vitamin D in the skin. However, vitamin D supplementation has shown little or no effect in the prevention of respiratory infections in adults. Thus, UVB light provides the immune system with essential benefits that are not produced by oral vitamin D supplements.