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Scientists uncover biological explanation behind why upper respiratory infections are more common in colder temperatures

Researchers at Mass Eye and Ear and Northeastern University have discovered a previously unidentified immune response inside the nose that fights off viruses responsible for upper respiratory infections. Further testing revealed this protective response becomes inhibited in colder temperatures, making an infection more likely to occur.

The new study, published December 6 in The Journal of Allergy and Clinical Immunology, offers the first biological mechanism to explain why viruses like the common cold, flu and COVID-19 are more likely to spike in colder seasons, according to the authors.

“Conventionally, it was thought that cold and flu season occurred in cooler months because people are stuck indoors more where airborne viruses could spread more easily,” said Benjamin S. Bleier, MD, FACS, director of Otolaryngology Translational Research at Mass Eye and Ear and senior author of the study. “Our study however points to a biological root cause for the seasonal variation in upper respiratory viral infections we see each year, most recently demonstrated throughout the COVID-19 pandemic.”

First-line defense in the nose

The nose is one of the first points of contact between the outside environment and inside the body and, as such, a likely entry point for disease-causing pathogens. Pathogens are inhaled or directly deposited (such as by the hands) into the front of the nose where they work their way backwards through the airway and into the body infecting cells, which can lead to an upper respiratory infection. How the airway protects itself against these pathogens has long been poorly understood.

That is until a 2018 study led by Dr. Bleier and Mansoor Amiji, PhD, Distinguished Professor of Pharmaceutical Sciences at Northeastern University, uncovered an innate immune response triggered when bacteria is inhaled through the nose: Cells in the front of the nose detected the bacteria and then released billions of tiny fluid-filled sacs called extracellular vesicles (or EVs, known previously as exosomes) into the mucus to surround and attack the bacteria. Dr. Bleier compares the release of this EV swarm to “kicking a hornets’ nest.”

The 2018 study also showed that the EVs shuttle protective antibacterial proteins through the mucus from the front of the nose to the back of it along the airway, which then protects other cells against the bacteria before it gets too far into the body.

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