Scientists discover why we get more colds and flu in winter

There’s a chill in the air and you all know what that means — it’s time for cold and flu season, when it seems like everyone you know is suddenly sneezing, sniffling or worse. It’s almost as if those pesky cold and flu germs pop up with the first blast of winter.

However, germs are present year-round — just think about your last cold of the summer. So why do people get more colds, flu and now Covid-19 when it’s cold outside?

In what they called a “breakthrough”, scientists discovered the biological reason why we have more respiratory diseases in winter — the cold air itself impairs the immune response that occurs in the nose.

“This is the first time we have a biological and molecular explanation for a factor in our innate immune response that appears to be limited by colder temperatures,” said rhinologist Zara Patel, professor of otolaryngology and head and neck surgery at Stanford. She was not involved in the new study.

In fact, reducing the temperature inside the nose by just 5°C kills nearly 50% of the billions of bacteria-fighting cells and viruses in the nostrils, according to the 2022 study published in The Journal of Allergy and Clinical Immunology.

“Cold air is associated with increased viral infection because you basically lose half of your immunity just from that small drop in temperature,” said study author Benjamin Bleier, director of translational research in otolaryngology at Massachusetts Eye and Ear and professor associate at Harvard Medical School, in Boston.

“It’s important to remember that these are in vitro studies, which means that although we are using human tissue in the laboratory to study this immune response, this is not a study carried out inside someone’s nose,” Patel said via email. mail. “The results of in vitro studies are often confirmed in vivo, but not always.”

A hornet’s nest

To understand why this occurs, Bleier, his team and co-author Mansoor Amiji, who chairs the department of pharmaceutical sciences at Northeastern University in Boston, went on a hunt for scientific detectives.

A respiratory virus or bacteria invades the nose, the main point of entry into the body. Immediately, the front of the nose detects the germ, well before the back of the nose notices the intruder, the team found.

At this point, the cells lining the nose immediately begin creating billions of single copies of themselves, called extracellular vesicles, or EVs.

“EVs cannot divide like cells, but they are like small versions of cells designed specifically to kill these viruses,” Bleier said. “EVs act as baits, so now when you inhale a virus, the virus sticks to these baits instead of sticking to cells.”

These mini versions are then expelled by the cells into the nasal mucus, where they prevent invading germs from reaching their destination and multiplying.

“This is one of, if not the only part of the immune system that goes out of the body to fight bacteria and viruses before they actually enter your body,” Bleier said.

Once created and dispersed in nasal secretions, the billions of EVs begin to spread the marauding germs, Bleier said.

“It’s like you kick a hornet’s nest, what happens? You may see some wasps flying around, but when you kick them, they all fly out of the nest to attack before the animal can enter the nest,” he said. “This is how the body eliminates these inhaled viruses so that they can never enter the cell.”

Increase in immune power

When under attack, the nose increases the production of extracellular vesicles by 160%, the study found. There were additional differences: the EVs had many more receptors on their surface than the original cells, thus increasing the virus-stopping ability of the billions of extracellular vesicles in the nose.

“Imagine the receptors as little arms that stick out, trying to grab viral particles as you breathe them in,” Bleier said. “And we found that each vesicle has up to 20 times more receptors on the surface, which makes them super sticky.”

The body’s cells also contain a viral killer called micro RNA, which attacks invading germs. However, EVs in the nose contained 13 times more micro RNA sequences than normal cells, the study found.

So the nose comes into battle armed with some extra superpowers. But what happens to these perks when cold weather arrives?

To find out, Bleier and his team exposed four study participants to 15 minutes of 4.4°C temperatures and then measured conditions inside their nasal cavities.

“What we found is that when you’re exposed to cold air, the temperature in your nose can drop to as low as -12°C. And that is enough to eliminate all three immunological advantages that the nose has,” said Bleier.

In fact, that bit of cold on the tip of the nose was enough to knock nearly 42% of the extracellular vesicles out of the fight, Bleier said.

“Similarly, you have almost half the amount of these killer micro RNAs inside each vesicle and you can have up to a 70% drop in the number of receptors in each vesicle, making them much less sticky,” he said.

How does this affect your ability to fight off colds, flu and Covid-19? It halves the immune system’s ability to fight respiratory infections, Bleier said.

Protecting the nose

Turns out the pandemic gave us exactly what we needed to help combat the cold air and keep our immunity up, Bleier said.

“Not only do masks protect you from directly inhaling viruses, but it’s also like wearing a sweater over your nose,” he said.

Patel agreed: “The warmer you can keep the intranasal environment, the better this innate immune defense mechanism will be able to function. Maybe one more reason to wear masks!”

In the future, Bleier hopes to see the development of topical nasal medications that build on this scientific revelation. These new pharmaceuticals “will essentially trick your nose into thinking you just saw a virus,” he said.

“By having that exposure, you’re going to have all these extra wasps flying around on your mucous membranes protecting you,” he added.