Why Does The Flu Spread More In The Winter?

In 2020, COVID-19 has taken over almost all media and research around the world. However, many forget that whatever it was, we still have other viruses in circulation, and even if they are less dangerous, they can still cause malaise or even exceptionally severe symptoms. One of these is the flu, which for some reason spreads more in the winter than in other seasons.

Indeed, many have wondered why the flu spreads most commonly in the fall and winter, and virtually disappears in the spring. In this article, we aim to provide an answer to this question.

Influenza: one clinical picture, several different forms

Influenza is an infectious disease caused by different types of influenza viruses. They are single-stranded RNA viruses that are covered by an outer lipid layer that gives them a typical rounded appearance. According to the World Health Organization, seasonal influenza can be found in 4 different types:

• Influenza viruses of type A, which are classified into subtypes on the basis of combinations of two proteins on their surface. All major influenza epidemics are caused by type A viruses.
• Type B viruses are not classified into subtypes, but viruses that are currently spreading around the world can be divided into two genera.
• Type C viruses are less common and usually cause mild infections. They have no epidemiological significance.
• Type D viruses mainly affect cattle and do not appear to cause infection or disease in humans.

The global incidence is estimated at about 20 percent, that is, 20 percent of the world’s population will develop it at some point. In addition, it is a selective pathology, as the incidence of influenza in certain population groups can be as high as 50 percent. Due to its epidemiological significance, influenza has been the subject of several studies.

Influenza and weather

A study published in Plos Pathogens explains the relationship between influenza and climatic factors as follows:

• The study utilized 20 experimental copies with guinea pigs. In some farms, i.e., several guinea pigs with the flu, while in other facilities, next to the sick, i.e., healthy guinea pigs. These spaces were connected to each other through the air.
• Both groups of samples were exposed to different relative humidity levels and temperatures.
• At constant temperature, it was found that the relationship between relative humidity and the spread of the flu was inverse. At 20% relative humidity, the majority of healthy guinea pigs became infected through aerosols delivered by diseased guinea pigs, whereas at 80% relative humidity, no spread of infection was observed among healthy guinea pigs.

• The study also found that diseased guinea pigs living at 5 degrees carried airborne particles  with viral loads more and for a longer time than those diseased guinea pigs that had been infected in 20-degree environments.

It seems that the main hypothesis is proved in this experiment: low temperatures and drought contribute to the spread of the virus. But why?

Low relative humidity

Various hypotheses attempt to explain why low relative humidity favors the spread of influenza:

• First, dry air can damage and degrade the nasal mucosa of the host organism, leaving them more vulnerable to respiratory viral infections such as influenza.
• Second, the stability of the virus in aerosols appears to vary with climatic humidity ; the virus would appear to remain active longer in less humid environments.

• Third, the virus may lose its ability to spread in environments with higher climate humidity. At high relative humidity, aerosols dispensed into a patient’s air can adhere more rapidly to ambient water molecules, increasing their volume, thereby also precipitating earlier than normal. This in turn would seem to reduce the speed and distance traveled by the virus.

Low temperatures

The explanation could be a little simpler in this case . At low temperatures, the nasal mucosa dries out when we breathe cold air. This would appear to create a more favorable microenvironment for the virus where it could replicate better. This, in turn, would turn into a greater viral load with each sneeze and cough, which would naturally also contribute to the spread of the virus to the immediate environment through aerosols delivered by the infected person.

Reasonable seasonality

As we have now seen, the seasonality of influenza has a clear scientific significance. In the northern hemisphere, this virus slowly begins to land in society at the turn of September-October and peaks roughly between December and February. This information is fully consistent with the results observed in the study presented above, as it is precisely these months that are cold and dry months in the northern hemisphere.

Fortunately, there is also a vaccine for the flu today. In Finland, the influenza vaccine is part of the national vaccine program for those people for whom influenza poses a significant threat to their health and for whose health the vaccine has a substantial benefit. Eligible for free influenza vaccination include people working in social and health care, pregnant women, children under the age of 7, and the population over the age of 65. The protection provided by an influenza vaccine lasts for about a year, so it is always important to renew it, especially in people at risk, before the start of a new flu season.