ISTITUTO NAZIONALE DI ASTROFISICA
IMAGE: DEVELOPMENT OF INFLUENZA-LIKE EPIDEMIC. Show more CREDIT: PAOLO BONFINI, UNIVERSITY OF CRETE
Why do most of the virus epidemics spread cyclically in the temperate regions of the world in autumn and winter? According to an interdisciplinary team of researchers from the Italian National Institute of Astrophysics, the University of Milan, the Regional Agency for the Environment in Lombardy and the Don Gnocchi Foundation, the answer is closely related to our sun: their theoretical model shows that both the prevalence and the The development of epidemics is strongly correlated with the amount of daily solar radiation that hits a certain place on earth at a certain time of year. The work of the Italian team was recently published in the iScience-Journal.
"Our model offers a simple answer to an important but still unsolved scientific question," says Fabrizio Nicastro, INAF researcher and PI of the work. "Why do many respiratory viral epidemics such as influenza in autumn and winter only develop cyclically in the temperate regions of the northern and southern hemispheres of the earth, even though they seem to be present at all times – albeit with a lower prevalence compared to the seasonal cycles in the temperate regions – in the equatorial belt? And what triggers and determines such seasonality? In our work we propose that the cause of the seasonality of airborne epidemics is precisely the same mechanism that causes the seasons on our planet: the amount of daily solar radiation on Earth. "
It is known that ultraviolet (UV) light can deactivate viruses and bacteria of all kinds. The solar UV light that reaches the earth must therefore have a certain disinfectant power on the exposed parts of the planet. The efficiency of UV deactivation of a given virus or bacterium depends on the virus or bacterium itself, but for a given location on earth it is undoubtedly greater when the solar radiation is stronger (summer) and lower when the solar radiation is weaker (winter ). Such a cyclicity of the solar disinfection effect with annual frequency can constructively resonate with another frequency typical of epidemics: the loss of the immunity of the host of the virus due to its antigenic shift / drift. The combination of these two mechanisms triggers the seasonality of epidemics on time scales that, depending on the antigen frequency, range between a few years and ten years.
The model proposed by the Italian researchers accurately reproduces the seasonality observed in different places on earth for epidemics with an intrinsic reproduction number (R0) less than about 2 – influenza typically has R0 ~ 1 – and can also model epidemics with a much larger intrinsic Reproduction number, like the current SARS-CoV-2 pandemic with R0? 3-4. These models predict intermittent initial high-intensity cycles that eventually stabilize (on time scales that depend on the antigen shift frequency) on seasonally synchronized annual cycles of medium intensity.
"From an epidemiological point of view, these models clear an important and long-standing mystery: why do influenza epidemics disappear every year when the number of people at risk is far from the number required to trigger the herd immunity mechanism?" Adds Mario Clerici, immunologist at the University of Milan and the Don Gnocchi Foundation.
“The Italian data on the SARS-CoV-2 pandemics can also be described precisely by our model – concludes Nicastro – but the predictive power of the model depends crucially (apart from the implementation of new restriction measures) on the exact UV-B / a lethal dose for the Covid-19 virus that will measure our collaboration. "
This work was published by the journal iScience in the article Forcing Seasonality of Influenza-like Epidemics with Daily Solar Resonance by Fabrizio Nicastro, Giorgia Sironi, Elio Antonello, Andrea Bianco, Mara Biasin, John R. Brucato, Ilaria Ermolli, and Giovanni Pareschi, Marta Salvati, Paolo Tozzi, Daria Trabattoni and Mario Clerici.