Why Upper Respiratory Infections Are More Common in Colder Temperatures

In a study with Harvard Medical School with the researchers at Mass Eye and Ear alongside Northeastern University, researchers discovered there was a previously unidentified immune response inside noses that fight off viruses that are responsible for upper respiratory infections. Upon more testing, it was revealed that this protective response is inhibited in colder temperatures which makes infections more likely to occur.

Why Do Cases of the Common Cold, Flu, and COVID-19 Tend To Spike More In The Colder Seasons?

It is often thought that cold and flu season occurs during the colder months because people are inside more often where airborne viruses can be spread easily. However, the root cause for these seasonal illnesses and their variations within the upper respiratory viral infections we see each year can be seen as demonstrated throughout the COVID-19 pandemic.

It all starts with the nose

Your nose tends to be the first point of contact when it comes to the spreading of germs that get inside the body as a main entry point for disease-causing pathogens. They are inhaled or even directly placed (i.e. by your hands) onto the front of your nose and make their way through your airways and body, infecting your cells. This is what leads to an upper respiratory infection. However, how your airways protect themselves against these pathogens has been wildly misunderstood.

There is an innate immune response that is triggered when the bacteria enters the nose. Cells that are in the front of your nose detect the bacteria and release billions of tiny fluid-filled sacs called extracellular vesicles into said mucus to surround and attach the bacteria.

In a recent study, it was discovered that the extracellular vesicles release protective antibacterial proteins through the mucus from the front of your nose going all the way back through the airway. This protects other cells against said bacteria before it gets too far into the body. In this study, they wanted to figure out if the immune response could also be caused by viruses inhaled through the nose. This is the source of some of the most common upper respiratory infections.

Testing the virus-fighting mechanism

Researchers studied and analyzed how nasal tissue samples and cells collected from the noses of patients undergoing surgery and healthy volunteers reacted to three viruses: a coronavirus and two rhinoviruses which cause the common cold. What they discovered was that each of these viruses was triggered by an extracellular vesicle swarm response from the nasal cells, despite using a signaling pathway different from the ones used to fight off the bacteria.

There was also a mechanism seen in response to viruses. When they were released, the extracellular vesicles acted like decoys and carried receptors that the virus would bind to instead of the nasal cells. The more decors there were, the more the extracellular vesicles can attach to the viruses in the mucus before the viruses have a chance to bind to the nasal cells. This suppresses the infection.

Later, it was tested on the effect of colder temperatures on the response. This is extremely relevant when it comes to nasal immunity given that the internal temperature of the nose is highly dependent on the temperature outside that is inhaled through the nose. Healthy volunteers who were in a room temperature environment were exposed to 39.9 F temperatures for 15 minutes and noticed that the temperature inside the nose fell by 9 Degrees F. Next, they applied this reduction in temperature to the nasal tissue samples and noticed a blunted immune response.  The number of extracellular vesicles secreted by the nasal cells decreased by almost 42% and the antiviral proteins within the extracellular vesicles were also changed.

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