Flu Syndrome: Why Does He Wake Up In Winter?
COVID-19 has monopolized the attention of the media and scientific research, yet there are still other viruses in circulation. They are certainly less dangerous, but they cause discomfort and symptoms that should not be overlooked. This is the case of the flu syndrome, a disease that mainly affects the winter season.
In spring and summer, with rising temperatures and better health conditions, it is natural to wonder why the flu has its greatest prevalence in autumn and winter. Let’s try to answer this question.
A clinical picture with multiple forms
Influenza is an infectious disease caused by several different viral strains. These are single-stranded RNA viruses covered with an external lipid layer which gives them a typical rounded appearance. According to the WHO, there are 4 influenza viruses:
- Influenza A. They are classified into subtypes based on the combination of two proteins present on their surface. All known influenza pandemics have been caused by type A viruses.
- Influenza B virus. They are not classified into subtypes, but those currently in circulation can be divided into two strains.
- Type C viruses are detected less frequently and usually cause mild infections. They have no great epidemiological importance.
- Type D viruses mainly affect cattle and do not appear to cause infection or disease in humans.
The global incidence is 20%, meaning that 20% of the world population sooner or later contracts one of these viruses. It is also a selective pathology, since in some categories it can reach an incidence equal to 50%. Due to its epidemiological importance, influenza is the subject of numerous studies.
The flu syndrome and the weather
A study published in the scientific journal Plos Pathogens explains the relationship between influenza and climatic factors:
- 20 experimental replicas were performed on guinea pigs. In some habitats there were several affected guinea pigs, in other contiguous habitats, but in air contact, there were healthy groups.
- All groups – with healthy and diseased guinea pigs – were subjected to varying levels of humidity and temperature.
- At constant temperature, relative humidity and influence transmission were inversely correlated. In the presence of 20% relative humidity, most healthy guinea pigs are infected through the respiratory droplets emitted by diseased guinea pigs. At 80% humidity, no contagion was observed.
- It was also observed that the respiratory droplets of the diseased guinea pigs contained a higher and more resistant viral load when the temperature was equal to 5 degrees centigrade. The situation changed by bringing the temperature to 20 degrees.
These experiments seem to confirm the main hypothesis: low temperatures and dry air favor the spread of the virus. But why?
Low relative humidity
There are several hypotheses that attempt to explain why low humidity favors the spread of flu:
- First, dry air could damage and deteriorate the host’s nasal mucosa, leaving it less protected from viral airway infections such as the flu.
- Second, the stability of viruses in respiratory droplets appears to vary with humidity. The virus remains active longer in less humid environments.
- Third, the virus may lose its ability to spread when humidity is higher. With a higher relative humidity, the droplets expelled by the sick subjects tend to adhere rapidly to the water molecules suspended in the environment; this fact causes them to increase in volume and precipitate earlier. This would decrease the speed and distance traveled by the viruses.
Flu syndrome and low temperatures
In this case, the explanation is a bit simpler: in the presence of low external temperatures, the nasal mucosa cools during inspiration. This would create a more favorable micro-environment for viruses that can replicate better. The result is a higher viral load with each sneeze, which thus favors the propagation of the pathogen.
A justified seasonality
We have found that the seasonality of the flu syndrome has a clear scientific explanation. In the northern hemisphere, influenza viruses begin to reappear in October, with a peak observed between December and February. These data agree perfectly with the research on display, as they are cold and dry months.
Fortunately, we have the flu shot available every year, so the problem is very small.
