Impact of Daily Solar Irradiation on Respiratory Viral Epidemics

Seasonal Patterns in Temperate Regions

Researchers have explored how daily solar irradiation affects the seasonal patterns of respiratory viral epidemics in temperate areas. It is widely recognized that respiratory viral outbreaks, including influenza, tend to peak during the autumn and winter months in both the northern and southern hemispheres. Conversely, equatorial regions experience respiratory viral epidemics throughout the year, albeit at lower prevalence rates. The underlying causes of these phenomena remain unclear.

Research Findings from the National Institute of Astrophysics

A team from the National Institute of Astrophysics at the University of Milan, Italy, proposes that the seasonality of respiratory viral outbreaks in temperate zones is associated with the cyclical variations in daily solar irradiation. They have developed a mathematical model to examine this hypothesis and recently published their findings in the journal iScience.

Understanding Respiratory Viral Epidemics

Previous theoretical models have focused on how changes within viruses, known as antigenic drift or shift, contribute to the emergence of new viral strains capable of evading the human immune system. However, these models do not fully account for the seasonal variations observed in different regions. Ultraviolet (UV) light is known to deactivate various viruses and bacteria, although its effectiveness can differ among organisms. The researchers suggest that solar UV light, which reaches the Earth’s surface, may exert germicidal effects that are more pronounced during summer months in temperate regions and diminished during winter.

Mathematical Model and Testing

The researchers proposed that the interplay between antigenic shift/drift and cyclical daily UV irradiation influences the seasonal dynamics of viral epidemics over timescales ranging from several years to decades. They developed a mathematical model based on these components and validated it using data from influenza viruses, which possess an intrinsic reproductive number (R0) of around 1, and data from SARS-CoV-2, which has an R0 between 3 and 4. The model effectively captured the seasonality and geographical dependence of respiratory viral diseases. Furthermore, it accounted for the variations in COVID-19 prevalence across different latitudes and accurately modeled the pattern of SARS-CoV-2 data observed in Italy over a six-month period.

Insights from the Study

“From an epidemiological perspective, these models illuminate a significant and long-standing question: why do influenza epidemics dissipate annually when the number of susceptible individuals remains far from what is needed to trigger herd immunity?” stated Dr. Mario Clerici, a lead author of the study. Co-lead author Dr. Fabrizio Nicastro noted that while the model effectively described the Italian SARS-CoV-2 data, its predictive capabilities depend on the specific UV lethal doses for the virus, a factor the team intends to investigate further.

References

1. Nicastro F, Sironi G, Antonello E, et al. Forcing seasonality of influenza-like epidemics with daily solar resonance. iScience 2020;23(10):101605. DOI: https://doi.org/10.1016/j.isci.2020.101605
2. Istituto Nazionale di Astrofisica. Press release, 2 Nov 2020. The role of the Sun in the spread of viral respiratory diseases. https://www.eurekalert.org/pub_releases/2020-11/inda-tro103020.php

Image by Pepper Mint from Pixabay.