The scientists recognized that the Sun plays an important role in the spread of viral respiratory diseases, including the COVID-19. At the University of Crete, the researchers found out why most viral epidemics can spread in temperate regions in the spring and fall. It turns out there is a close connection with the Sun. The development and spread of epidemics correlate with the amount of daytime radiation.
It falls into certain zones on the Earth during these periods of the year. A researcher Fabrizio Nicastro says the hypothesized pattern provides an answer to an important scientific question. Viral epidemics similar to influenza develop cyclically in winter and autumn in regions where the climate is traditionally temperate. In their work, the experts suggested that the reason for the seasonality of epidemics may lie in the mechanism of the spread of infection by air.
That mechanism is similar to the one that causes seasons on our planet. As it is known, ultraviolet light is able to kill viral infections and bacteria. And reaching the Earth, it has the ability to disinfect in open areas of the planet. The efficiency of the radiation that neutralizes viruses depends on the specific virus or bacteria, but for a certain place on the planet it is higher when the solar radiation is stronger. As a rule, it is summer, and in winter the ability to radiate is significantly weakened.
This cyclicality can resonate with a different frequency that is typical of epidemics. A person may risk loss of immunity due to antigenic shift. The combination of these two mechanisms causes seasonality of epidemics on timescales ranging from several years to tens of years, depending on the frequency of occurrence of antigens.
The model that the scientists created accurately reproduces the seasonality that is observed in different parts of the world. It demonstrates epidemics with an internal reproductive number, simulates diseases with an internal reproductive number, such as the COVID-19.
The model shows intermittent initial cycles of high activity, which shift towards seasonally synchronized annual cycles of moderate activity. The scientists believe that the models created demonstrate the answer to a long-standing mystery why epidemics arise and then disappear.