Why upper respiratory infections are more common in cold weather

Viruses circulate all year round. The fact that there are more people with colds in the cold season has not only to do with staying indoors - but with the temperature in our nose.

Why are colds more common in low temperatures?

A recent study published in the Journal of Allergy and Clinical Immunology provides a direct quantitative mechanistic explanation for seasonal variations in the prevalence of upper respiratory tract infections.
There is growing evidence that temperature and humidity independently or jointly influence susceptibility to respiratory viral infections.
Cold exposure decreases nasal antiviral immunity mediated by extracellular vesicles

A cold temperature inhibits the viral defence in the nose

The nose is often the entry point for pathogens. At this first point of contact, numerous important immune reactions are set in motion to defend against them. One of these is the release of swarms of extracellular vesicles (EVs) from the epithelial cells of the nose into the mucus. These EVs contain various antimicrobial and immunomodulatory substances. Of particular importance here is the delivery of antiviral agents, such as microRNAs, to neighbouring or distant recipient cells. EVs can also have a direct neutralisation of virions by "catching" them away with their surface receptors and thus preventing their binding to the epithelial cells.

A new study has shown that these vigorous non-specific defence responses are greatly reduced by lower temperatures in the nose, specifically the EV swarm response to contact with viruses, which is triggered by a different signalling pathway (TLR3-mediated) than when bacteria are inhaled.

For this purpose, healthy test subjects were exposed to temperatures of around 4 degrees Celsius at 90% humidity for 15 minutes. The endoscopically measured internal temperature in the nasal cavity dropped by an average of 5°C as a result. The cooled nasal cells released 42% fewer EVs under these conditions than at warmer temperature and medium humidity (23°C and 57%). The individual EVs also showed reduced miRNA storage and antiviral binding affinity in cold conditions.

Conclusion for medical practice

So there might be more to the old saying we started with than meets the eye. Keep your nose warm in winter. A drop in temperature of just 5 degrees Celsius in the nose seems to lower the local immune response, which could help explain the accumulation of illnesses in the cold season.

Based on the findings of the study, the researchers hope that, on the one hand, new therapies for viral infections of the respiratory tract could be developed - for example, a nasal spray that increases the amount of EV in the nose or increases the number of binding receptors on the vesicles - and, on the other hand, potential adverse effects of antiviral drugs could be reduced.

Sources:

In German only: @NatGeoDeutschland. Überraschende Studie: Warum wir im Winter öfter erkältet sind. National Geographic (2022).
Huang, D. et al. Cold exposure impairs extracellular vesicle swarm–mediated nasal antiviral immunity. Journal of Allergy and Clinical Immunology 0, (2022).

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