Why a cold undermines our defense against respiratory infections

The father of medicine, the Greek physician and philosopher Hippocrates, wrote in 400 BC: “Because winter will solve the diseases of summer, and the coming of summer will eliminate the diseases of winter.”

His text is considered the first to be a treatise on the seasonal difference in a disease, that is, influenza. But the question of why some diseases appear at certain times of the year was a mystery that scientists could only solve in the modern era.

Although not all of the puzzle pieces have been found for the phenomenon, researchers now believe they have an understanding of the seasonal cycle of bacilli and their hosts and the conditions in which they live. This cycle undergoes complex changes that make people more or less vulnerable to disease.

“If you ask five influenza experts what they think is the main additional factor, you have a reasonable chance of getting five different answers,” said Dylan Morris, a postdoctoral fellow at the University of California, Los Angeles who studies ecology and ecology. He studies the evolution of the virus with a smile.

However, scientists have discovered something, which is that changing the seasons not only affects the physical structure of viruses, but also the defense mechanisms that our bodies use against them. Especially in winter, dry and cold air and lack of sunlight have a negative effect on the body’s ability to ward off respiratory infections such as influenza or the SARS-CoV-2 virus.

Coronavirus in the winter

Given their experiences with the common cold and seasonal flu, health care experts have warned of the possibility of a sharp rise in the number of COVID-19 cases in the winter, when cold weather causes people to stay home often, as airborne transmission of the Corona virus is more common. More admired. Unfortunately, their concerns have been confirmed, because in the United States and other Western countries, the number of infections, hospitalizations and deaths from COVID-19 has risen to record levels in recent weeks and does not appear to be easing at the moment.

Morris and colleagues recently published a preliminary version of a study looking at one of the contributing factors to this rise in infections, which is how the stability of the coronavirus has changed due to cold and humidity.

They found that at very high and very low temperatures and humidity levels, the virus remains more stable and contagious for longer. At lower temperatures, the chemical reactions are slower, which also slows down the viruses’ breakdown. This means that the virus stays for longer in the droplets that are expelled from the respiratory system.

At lower humidity, smaller droplets form, which makes it easier for the virus to come into contact with other chemicals in the droplets and thus is turned off. But this is only true to some extent: When the droplets get very small, the natural salts in the fluids we exhale begin to crystallize and trap the virus particles, preserving them until they enter the airways of a new host. Melt again and wake up.

Unfortunately, winter is a time of low temperatures and low humidity. Other variables, such as people staying home often and not getting enough sunlight, also contribute to the problem.

“All of these factors point in the same direction and are reasons to expect respiratory infections in winter to be more difficult,” Morris says.

Winter conditions also affect the infection’s other protagonist: the infected body.

Fresh cold winter air

Akiko Iwasaki, an immunologist at Yale University and a researcher at the Howard Hughes Medical Institute, says that breathing in cold, dry air triggers changes in our immune system.

In a study published in 2015, Iwasaki and her colleagues showed that epithelial cells in the airways of mice produce less interferons when the weather is cooler. Interferons are a class of proteins that trigger the alarm in viral infections and stimulate immune cells to take action to stop the virus.

“Since that study came out, I have asked my kids to wear a scarf over their noses – and now, of course, a mouth mask – because it keeps the nose temperature higher,” says Iwasaki.