Why colds and flu viruses are more common in winter

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A chill is in the air, and you all know what that means — it’s cold and flu season, when everyone you know suddenly seems to be sneezing, sniffling, or worse. It’s almost as if those pesky cold and flu germs are swirling with the first blast of winter weather.

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

In what researchers are calling a scientific breakthrough, scientists behind a new study may have found a biological reason why we get more respiratory illnesses in the winter. It turns out that the cold air itself damages the immune response that occurs in the nose.

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

In fact, lowering the temperature in the nose by just 9 degrees Fahrenheit (5 degrees Celsius) kills nearly 50% of the billions of cells in the nostrils that fight viruses and bacteria, according to a study published Tuesday in Journal of Allergy and Clinical Immunology.

“Cold air is associated with increased viral infection because you’ve essentially lost half of your immunity just because of that small drop in temperature,” said rhinologist Dr. Benjamin Bleier, director of otolaryngology at Massachusetts Eye and Ear and an associate professor at Harvard Medical School in Boston.

“It’s important to remember that these are in vitro studies, which means that while human tissue is used in the lab to study this immune response, it’s not a study done inside someone’s actual nose,” Patel said in an email. “Often findings in vitro are confirmed in vivo, but not always.”

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

A respiratory virus or bacteria attacks the nose, the main point of entry into the body. The team found that the front of the nose immediately detects the germ, long before the back of the nose becomes aware of the intruder.

At that point, the cells lining the nose immediately start making billions of simple copies of themselves called extracellular vesicles, or EVs.

“EVs can’t divide like cells, but they’re like little mini versions of cells specifically designed to go and kill these viruses,” Bleier said. “EVs act as decoys, so now when you inhale the virus, the virus sticks to those decoys instead of sticking to the cells.”

These “Mini Me” cells are then released into the nasal mucus (yes, nasal mucus), where they stop invading germs before they reach their destinations and multiply.

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

Once formed and dispersed into nasal secretions, the billions of EVs then begin to swarm the marauding germs, Bleier said.

“It’s like kicking a hornet’s nest, what happens? You might see a few hornets flying around, but when you kick it, they all fly out of the nest to attack before that animal even enters the nest itself,” he said. “It’s the body’s way of cleaning up these inhaled viruses so they can’t enter the cell at all.”

When attacked, the nose increases the production of extracellular vesicles by 160%, the study found. There were additional differences: EVs had many more receptors on their surface than the original cells, enhancing the virus’ ability to stop the billions of extracellular vesicles in the nose.

“Just think of the receptors as little arms sticking out, trying to grab hold of viral particles as you inhale them,” Bleier said. “We found that each vesicle has up to 20 times more receptors on the surface, which makes them super sticky.”

Cells in the body also contain a viral killer called micro RNA, which attacks invading germs. However, the EV in the nose contains 13 times more micro RNA sequences than normal cells, the research showed.

So the nose comes into battle armed with some extra superpowers. But what happens to those benefits when the cold weather hits?

To find out, Bleier and his team exposed four study participants to temperatures of 40 degrees Fahrenheit (4.4 degrees Celsius) for 15 minutes, then measured conditions in their nasal cavities.

“We found that when you’re exposed to cold air, the temperature in your nose can drop as much as 9 degrees Fahrenheit. And that’s enough to essentially knock out all three of the immune benefits that the nose has,” Bleier said.

In fact, that little bit of cold on the tip of the nose was enough to knock almost 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 on each vesicle, making them much less sticky,” he said.

What does this do to your ability to fight off colds, flu and Covid-19? It cuts your immune system’s ability to fight respiratory infections in half, Bleier said.

“It turns out that the pandemic has given us exactly what we need to fight the cold air and keep our immunity high,” Bleier said.

“Not only do masks protect you from directly inhaling the virus, but it’s 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 can work. Maybe another reason to wear masks!”

In the future, Bleier expects to develop topical nasal medications that build on this scientific discovery. These new drugs will “essentially trick the nose into thinking it’s just seen a virus,” he said.

“Because of that exposure, you’re going to have all these extra hornets flying around in your lining protecting you,” he added.