Why do viruses spread in the winter? Researchers say blame the sun

ROME, Italy — Everyone knows when flu season arrives. Once the winter coats come out and the flu shots start flowing, it’s only a matter of time before people start getting sick. So why does flu season arrive with such clockwork precision? A study finds a lot of it has to do with the sun, making dark winter nights primetime for epidemics.

Researchers from Italy say most viruses spread in fall and winter due to the lack of solar radiation striking the Earth in more temperate regions. Their scientific models reveal the frequency and evolution of epidemics have a strong connection to the amount of sunlight that hits a certain location throughout the year.

“Our model offers a simple answer to an important, yet still unsolved, scientific question”, says Fabrizio Nicastro of the Italian National Institute for Astrophysics in a media release.

“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.”

UV light from the sun kills germs

Researchers say it’s commonly accepted that ultraviolet (UV) light can deactivate many varieties of viruses and bacteria. Their study assumes UV rays reaching the Earth must therefore be able to disinfect surfaces as well.

While UV’s ability to destroy viruses depends heavily on what bacteria the light is dealing with, study authors conclude that it’s helpful to have more sunlight during the summer to do this job. This however makes weaker radiation in the winter less effective at irradiating germs. Researchers say this effect mixes with another problem of the season, a loss of immunity in the virus’s host. All this builds up into a regular opportunity for epidemics to strike.

The schedule of spreading viruses

The Italian models look at the reproductive number (R0) tied to illnesses; a number which notes how many potential infections each person a virus is capable of creating. An influenza patient typically has an R0~1. In the case of the coronavirus pandemic, researchers score it around R0?3-4.

The results predict that even initial virus cycles with a high-intensity spread eventually stabilize over time. They fall into a seasonally synchronized cycle with a less intense impact.

“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?” says Mario Clerici from 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,” Nicastro concludes. “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.”

The study appears in the journal iScience.

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