Dolly Dholakia

For many decades, different theoretical and practical studies and models pinpoint that prevalence and evolution of the spread of viral respiratory diseases are somehow related to daily solar irradiation that hits a given location on the Earth at a given time of the year. Why do many viral respiratory epidemics, such as influenza, grow cyclically only in the temperate regions of the northern and southern hemispheres of the globe during autumn and winter, although they appear to be present in the equatorial belt at all times, though with a lower incidence relative to the seasonal cycles in the temperate regions? And what causes such seasonality and decides it?

It is well known that viruses and bacteria of several different kinds can be deactivated by ultraviolet (UV) light. Solar radiation dictates the Spread of Viral Respiratory Diseases Therefore, in the exposed areas of the globe, the solar UV light that enters the earth must have some disinfecting capacity. The efficiency of the UV deactivation of a particular virus or bacterium depends on the virus or bacterium itself, but for a given location on Earth, it is certainly greater when the solar irradiation is higher (summer) and lower when the solar irradiation is weaker (winter). This cyclicality of solar disinfecting action, with annual frequency, is capable of resonating constructively with another epidemic-typical frequency: the loss of immunity of the host-virus due to its antigenic shift/drift. The combination of these two mechanisms causes the seasonality of epidemics, depending on the antigenic frequency, on timescales ranging from a few years to tens of years.

These models explain an important and long-standing mystery from an epidemiological point of view: why influenza epidemics disappear every year when the number of susceptible individuals is still very far from what is needed to trigger the herd immunity mechanism.