vector background of viruses and bacteria for medicine

Contributed by Dawn Yeomans, Ph.D., Hygiene Sciences & Partnerships Senior Advisor, GOJO Industries

As you may know, winter is prime time for the circulation of respiratory viruses like those that cause colds and the flu. But have you ever wondered why we are more likely to catch colds when the weather cools down? One reasonable explanation is that we tend to spend more time inside during the day when it’s cold and dreary outside – in schools, offices, and other indoor environments – which means lower ventilation levels and closer contact with each other. These factors mean that once one person catches a cold or another respiratory virus like those that cause the flu or COVID, it can more easily spread to others.

While this is a very good (and valid) reason, it appears as though there may be more to the story – and the answers are driven by our own human biology. For several years now, scientists have recognized that the temperature around us actually modifies our immune responses and risk of getting sick! As you likely know, your immune system is your primary defense against outside invaders. These include germs such as bacteria and viruses. Our body’s immune system is pretty complex – it’s made up of many different organs, cells, and proteins that work together to fight germs.

It all begins with our nose

While scientists have appreciated that temperature plays a role in exposing us to infections, an understanding of exactly how this happens (aka “What is the biology behind it?”) has eluded us until very recently. And one of the key pieces to the puzzle is understanding the biology that happens in our noses! Our noses are one of the first gateways between the outside world and our inner bodies. Because of this, our upper respiratory tracts (think noses, mouths, and throats) are often the first part of our bodies exposed to germs through inhaling droplets containing viruses and bacteria from the coughs and sneezes of infected individuals around us.

In its role as the gatekeeper to our bodies, the nose also is our first line of defense against germs; the inside of the nose (hairs, mucus, and all!) serves as a physical barrier to prevent germs from entering the body. The cells in the nose also serve as conductors of our body’s immune responses – turning it on if needed and off when the threat is gone. Our noses actually have biological “sensors” that detect when we are exposed to viruses and ramp up the cells of our immune system to mount an attack. 

The biology of our nose

In the past few weeks, we’ve gained a better understanding of the biology that causes this to happen – and it all starts with these cells in our noses. When we are exposed to viruses and our immune systems are activated, scientists have discovered that biological “sacs” in our noses swarm to the area where viruses are found as an emergency 9-1-1 antimicrobial response. These sacs – called extracellular vesicles, or “EVs” – come in different sizes and carry different contents. EVs act in two important ways to protect us from germs:

1. EVs themselves can directly attach to the virus and prevent it from infecting our cells. 

2. Some EVs carry antiviral materials in their sacs -— called microRNAs or miRNAs — which, when released from the EV, target the virus, inactivating it.  

Through these two actions, EVs protect us from getting an infection.

The impact of winter

So, how does cold weather fit in? What happens when the cells in our nose get cold and how does this increase our risk of infection? As you know anecdotally – and scientists have studied in detail – the nose itself is very sensitive to changes in temperature. Cold, dry air can irritate the inner lining of your nose, and as a result, your nasal glands produce more mucus to keep it moist. This is what causes your nose to drip and “run” in the cold weather. Other cold-sensing proteins on the cells of your nose – called “transient receptor proteins” or “TRPs” – are triggered by the cold and can cause other sensations like burning, tickling, and feelings of constriction that we’ve all experienced in the cold weather.

A new study sheds light

Last week, there was significant media coverage from outlets like Forbes, CNN, and the New York Post8,9,10 reporting a newly published study by scientists at Harvard and Northwestern Medical Schools11 that taught us even more about the links between cold weather, infections, and the role of the nose. Scientists discovered that cold air actually prevents the “sacs” (aka “EVs”) from working their magic to protect us from infection! When scientists reduced the temperature of cells they were growing in the lab or exposed humans to lower temperatures, they found that EVs didn’t “swarm” as they should to the site of infection, and those that were there didn’t work properly. A special miRNA called miR-17 was missing from the mix, which is critical for the inactivation of the virus that causes the common cold.

So, the science boils down to the cells and “sacs” in your nose not functioning their best to fight germs in cold temperatures. And what that means for all of us is a greater risk of viral infections that cause colds, the flu, COVID, and other respiratory illnesses in the winter months. For us science geeks – this is a pretty cool finding! But for everyone else, these discoveries are so interesting and novel that it’s not surprising that the news and popular media are talking about it too!

What you can do to stay healthy

While there’s not much you can do about biology (or the weather!), you can take simple actions to prevent the spread of germs:

1 Practice good respiratory etiquette (i.e., cover coughs/sneezes)

2 Wash or sanitize your hands often

3 Disinfect hard surfaces that may be contaminated with germs

4 Stay home if you’re feeling ill

If you want to learn more about this new research, read the Harvard and Northeastern articles or the full scientific article in the Journal of Allergy and Clinical Immunology here.