The flu is an infectious disease caused by influenzavirus A either influenzavirus Bboth RNA viruses of the family Orthomyxoviridae. This pathology presents a clear seasonal pattern, as it usually shows epidemiological peaks at the end of autumn and during winter in temperate climates.
But, Why does the flu appear in winter? Various studies try to answer this question which, although it may not seem like it, still does not have as clear an answer as the majority of the general population might believe. Continue reading if you want to know more.
Why does the flu appear more in winter? A multifactorial response
First of all, to understand the seasonal variation of this disease, it is necessary to categorize it on several fronts.
The World Health Organization (WHO) shows us that There are several types of viruses that cause seasonal flu These are the following:
The influenzavirus A They are classified according to structural surface proteins, hemagglutinin (HA) and neuraminidase (NA). The subtypes currently circulating in humans are A(H1N1) and A(H3N2), and all pandemics recorded so far regarding influenza have been caused by these influenzaviruses. These are the most aggressive pathogens of the types that cause the pathology.
Instead, the influenzavirus B They are less common and less aggressive than those mentioned previously. Their low mutation rate and their unique ability to infect humans and seals (in contrast to type A, which have many more hosts) make these viral variants less likely to cause an epidemic. Those currently circulating can be divided into two lineages B/Yamagata and B/Victoria.
Finally, we can also find influenzavirus types C and D, although they are very rare and their epidemiological importance is reserved for specific outbreaks.
So, as we have seen, the world of influenza is much more extensive than what could initially be imagined at a merely microscopic level. Even so, there is much more to investigate when it comes to infectious dynamics.
The overall incidence of influenza (the number of new cases of the disease in a specific time period in a given population) is 10 to 20% of the general population It is said quickly, but this means that one fifth of all people on Earth have a flu condition at the time you read these lines.
The population groups considered “at risk”, according to the WHO, are pregnant women, babies under 59 months of age and patients with chronic diseases or immunosuppression (as is the case of HIV-positive people).
Once we have dissected the typology of the disease and how it affects the general population, it is time to answer the following question: Why does the flu appear in winter?
The reason for seasonality
It is necessary to emphasize, first of all, that It is not entirely clear why the flu is seasonal From now on we will move on assumptions, of course supported by scientific studies, but which in no case can be considered absolute realities.
Furthermore, as many professionals say, “sometimes science is based more on asking the right questions than on detecting undeniable realities.”
Below, we show you 3 possible explanations for flu seasonality.
1. Seasonal variations in contact
It is clear that the behavior of the hosts (in this case, humans) must play an essential role in the spread of the disease in any epidemiological process.
Various studies hypothesize that The fact that people gather more in public spaces during the winter can promote the spread of the flu These are based on the reality that, for example, in the United States, any monitored person spends an average of 1-2 hours more indoors during cold weather episodes.
To provide more evidence for what was previously reported, other sources maintain that flu transmission in children during vacations is reduced by almost 30% compared to schooling periods.
Viruses are naturally transmitted with the secretions of watery microparticles present in coughs and sneezes. The more people accumulate in a sealed, unventilated space, the easier it will be to inhale contaminated emissions
Although all of this evidence seems to show us that it is “unfathomable” that variations in human behavior during the winter are the cause of winter flu peaks, there is no empirical data that links both factors in a completely reliable way.
2. Seasonal variations in virus survival
The time that the virus remains in the environment after its secretion is essential to quantify its epidemiological success. The longer it can survive in the external environment, the more likely it is that a healthy person can become infected, right?
According to this reasoning, 4 out of 6 medical studies have shown that the survival of the influenza virus increases the lower the relative humidity of the environment. This could be possible because The higher the humidity, the more likely the virus is to attach to water vapor particles in the air which would cause its early precipitation, instead of being able to infect another host.
The relationship between temperature and viral effectiveness is not so clear or proven, but preliminary studies with laboratory animals have shown that, at 30 degrees of ambient temperature, contagion is completely prevented. This is why it is also believed that the structure and effectiveness of influenzaviruses could be reduced in hot environments.
3. Seasonal variations in the host immune system
As expected, just as important as the tools of virus transmission are the host’s defense barriers in the face of an epidemiological process. Therefore, we cannot forget that We ourselves also change both internally and externally depending on the time of year
Here we are moving into much more intricate terrain, since the effectiveness of the host’s immune system is completely multifactorial and mediated by complex processes that we cannot cover in a few lines. For example, during winter, the combined effect of cold and dryness produces a loss of moisture in the nasal mucosa, which could facilitate the entry and multiplication of the virus.
Another factor could be the lack of sunlight, as this translates into a decrease in the levels of vitamin D in the host’s body, which can weaken the effectiveness of the host’s immune system. In relation to this topic, other studies also explore that food availability in certain regions during winter is lower, which would translate into a lack of essential nutrients compared to other times of the year.
Again, it should be noted that the body’s immune system depends on many factors, and for every study that shows a correlation between one of them and the presence of the flu, it is likely that another does not find clear evidence.
Conclusions
As we have seen in these lines, to the question of “why does the flu appear in winter?” We cannot give a clear answer that applies in 100% of cases. For example, in certain climates the incidence of influenza shows epidemiological peaks in the rainy season, which does not make sense if we take into account that relative humidity has been seen to reduce the effectiveness of virus transmission.
So what is more important, the survival of the virus, the behavior of humans, or the host’s immune system? Unfortunately, we cannot give you an answer yet. It is possible that a synergistic effect occurs between all the variations that we have mentioned, which would promote the appearance of seasonal flu epidemiological peaks.
