The Role of the Sun in the Spread of Viral Respiratory Diseases Like the Flu and COVID-19

Evolution of Flu-Like Epidemic

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Evolution of Influenza-like epidemic. Credit: Paolo Bonfini, University of Crete

Why do most viral upsurges spread out cyclically in fall and winter season in the world’s temperate areas? According to an interdisciplinary group of scientists of the Italian National Institute for Astrophysics, the University of Milan, the Lombardy local firm for the environment and the Don Gnocchi Foundation, the response is totally associated to our Sun: their theoretical design reveals that both the frequency and development of upsurges are highly associated with the quantity of everyday solar irradiation that strikes an offered place on the Earth at an offered time of the year. The work of the Italian group was just recently released in the iScience journal.

“Our model offers a simple answer to an important, yet still unsolved, scientific question,” states Fabrizio Nicastro, INAF scientist and PI of the work. “Why do many viral respiratory epidemics, such as influenza, develop cyclically during autumn and winter only in the temperate regions of the globe’s northern and southern hemispheres, while they seem to be present at all times – albeit with 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 what causes the seasonality of airborne-transmitted epidemics is exactly the same mechanism that causes seasons on our Planet: the amount of daily solar irradiation on the Earth.”

It is popular that ultraviolet (UV) light has the ability to shut down infections and germs of several kinds. The solar UV light that reaches the Earth should for that reason have some sanitizing power on the exposed parts of the Planet. The effectiveness of the UV deactivation of a specific infection or germs depends upon the infection or germs itself, however, for an offered place on Earth, it is certainly higher when the solar irradiation is more powerful (summertime) and lower when the solar irradiation is weaker (winter season). Such cyclicality of the solar sanitizing action, with yearly frequency, has the ability to constructively resonate with another frequency normal of upsurges: the loss of resistance of the infection’s host due to its antigenic shift/drift. The mix of these 2 systems sets off the seasonality of upsurges, on timescales that vary from a couple of years to 10s of years, depending upon the antigenic frequency.

The design proposed by the Italian scientists recreates the seasonality observed in various areas of the Earth precisely for upsurges with an intrinsic reproductive number (R0) lower than about 2 – an influenza normally has R0~1 – and is likewise able to design upsurges with a much bigger intrinsic reproductive number, such as the present SARS-CoV-2 pandemic with R0≈3–4. These designs anticipate high-intensity periodic preliminary cycles, which ultimately support (on timescales that depend upon the antigenic-shift frequency) onto seasonally-synchronized, moderate-intensity yearly cycles.

“From an epidemiologic point of view, these models clarify an important and long-standing mystery: why do influenza epidemics disappear every year when the number of susceptible individuals is still very far from that needed to trigger the herd immunity mechanism?” includes Mario Clerici, Immunologist at the University of Milan and the Don Gnocchi Foundation.

“The Italian data of the SARS-CoV-2 pandemics can also be described accurately by our model – concludes Nicastro – but the predictive power of the model depends critically (other than on the implementation of new restriction measures) on the exact UV-B/A lethal doses for the Covid-19 virus, which our collaboration is about to measure.”

Reference: “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, Giovanni Pareschi, Marta Salvati, Paolo Tozzi, Daria Trabattoni and Mario Clerici, 22 September 2020, iScience.
DOI: 10.1016/j.isci.2020.101605

This work has actually been released by the iScience journal, in the short 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, Giovanni Pareschi, Marta Salvati, Paolo Tozzi, Daria Trabattoni, and Mario Clerici.

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