Why viruses have it easier in winter

The name alone suggests that colds have something to do with the cold season. In winter they accumulate massively - as can also be seen again at the moment. This applies to conventional corona and rhinoviruses as well as to Sars-CoV-2 and flu pathogens. Scientists report in the "Journal of Allergy and Clinical Immunology" about a possible biological reason why such viruses could have it easier in winter. A cooling of the nasal cavity weakens mechanisms with which the nasal mucosa protects the body from viral cold pathogens. According to one expert, however, the effect does not play a particularly important role.

So far, factors related to physical characteristics and human behavior have been known above all: In winter, people spend more time indoors, where viruses released by sick people are more concentrated than outdoors, as the immunologist Percy Knolle from the Klinikum rechts der Isar of the Technical University of Munich explained. In addition, there is less ventilation during the cold season. Outside, on the other hand, the UV radiation from the sun is significantly lower than in summer. "UV light is the most critical for viruses, it inactivates them."

Experts also assume that the body's own mechanisms also play a role. The team led by Benjamin Bleier from Harvard Medical School has now identified such a factor: the so-called extracellular vesicles (EV), of which fewer are released at cooler temperatures. A group led by Bleier had already reported in 2019 that cells in the nose emit large amounts of such tiny particles with antibacterial properties into the nasal mucus when they come into contact with bacterial pathogens. The researchers have now clarified that billions of such particles are also released when there is contact with viral pathogens such as rhinoviruses or corona viruses.

"Many respiratory viruses initiate infection in the nasal cavity, which is the first region of contact with inhaled pathogens and is very sensitive to changes in ambient temperature," the group writes. For two rhinoviruses (RV-1B and RV-16) and a coronavirus (HCoV-OC43), she tested how the temperature affects the reaction of the nasal mucosa - especially the EV release.

When the room temperature dropped to 4.4 degrees, the temperature in the nasal cavity fell from 37 to about 32 degrees after 15 minutes, as the researchers report. As a result, the number of distributed EVs fell by almost half (42 percent). In addition, the antiviral properties of their ingredients would have suffered. The scientists conclude that cold can make an organism more susceptible to infections in this way.

The Munich immunologist Percy Knolle, who was not involved in the analysis, sees no significant factor in the mechanism for the winter high in respiratory diseases. The experimental approach is interesting - but other factors are probably much more important, he said. A drop of around half is not much when it comes to viruses. "There may only be hundredths or thousandths left for a significant impact."

In addition, one aspect had been completely forgotten: "The virus also has to struggle with the cold." Not only the processes in the host's nasal mucosa, but also the processes of infection were slower, explained Knolle. "That wasn't even taken into account in the study." No one has to worry about catching colds more often because fewer vesicles are released in an apartment that was less heated as a result of the energy crisis.

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