Why does the flu appear in winter?

A summary of the possible causes why flu viruses are more active in winter.

Influenza is an infectious disease caused by the influenzavirus A o influenzavirus Bboth RNA viruses of the family Orthomyxoviridae. This pathology has a clear seasonal pattern, as it usually shows epidemiological peaks in late autumn and during the winter in temperate climates.

But, why does influenza appear in winter? Several studies try to answer this question which, although it may not seem so, still does not have as clear an answer as most of the general population might believe. Read on if you want to know more.

Why does the flu appear more in winter? A multifactorial answer

First, to understand the seasonal variation of this disease, it is necessary to categorize it on several fronts.

The World Health Organization (WHO) shows that there are several types of viruses that cause seasonal influenza. there are several types of viruses that cause seasonal influenza.. These are as follows:

The influenzavirus A 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 reported to date are A(H1N1) and A(H3N2).and all influenza pandemics recorded to date have been caused by these influenzaviruses. These are the most aggressive of the pathogens that cause the disease.

On the other hand, the influenzavirus B are less common and less aggressive than those mentioned above. Their low mutation rate and unique ability to infect humans and seals (in contrast to type A, which has many more hosts) make these viral variants less likely to cause epidemics. The currently circulating viruses can be divided into two lineages B/Yamagata and B/Victoria.

Finally, we can also find influenzaviruses type C and D influenzaviruses, 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 might at first be imagined on a purely microscopic level. Still, there is much more to investigate in terms of infective dynamics.

How is it distributed in the population?

The overall incidence of influenza (number of new cases of the disease in a given time period in a given population) is 10 to 20% of the general population.. It is early days, but this means that one fifth of all people on Earth are suffering from influenza at the time you are reading these lines.

The population groups considered "at risk", according to the WHO, are pregnant women, infants under 59 months of age and patients with chronic diseases or immunosuppression (such as HIV-positive patients).

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 point out, first of all, that the reason for the seasonality of influenza is not entirely clear.. From now on we will move in assumptions, of course supported by scientific studies, but which in no case can be considered as absolute realities.

Moreover, as many professionals say, "sometimes science is more about asking the right questions than detecting undeniable realities".

Here are 3 possible explanations for flu seasonality.

Seasonal variations in contact

It is clear that host behavior (in this case, humans) must play an essential role in the spread of the disease in any epidemiological process.

Several studies hypothesize that the fact that people become more crowded in public spaces during the winter may promote the spread of influenza.. 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 add further evidence to the above, other sources claim that influenza transmission in children during vacations is reduced by almost 30% compared to school periods.

Viruses are naturally transmitted by watery microparticle secretions present in coughs and sneezes. The more people accumulate in an airtight space without ventilation, the easier it is to inhale the contaminated emissions..

Although all this evidence would seem to show that variations in human behavior during the winter are the "given" cause of winter flu peaks, there is no empirical data linking the two factors in a completely reliable way.

2. Seasonal variations in virus survival

The length of time that the virus remains in the environment after its secretion is essential to quantify the epidemiological success of the virus. to quantify the epidemiological success of the virus. The more 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 itself to water vapor particles in the air, causing it to precipitate.This could be possible because the higher the humidity, the more likely the virus is to adhere to water vapor particles in the air, causing it to precipitate early, rather than infect another host.

The relationship of temperature to viral efficacy is not as clear or proven, but preliminary studies with laboratory animals have shown that at 30 degrees Celsius ambient temperature, transmission is prevented altogether. This is why it is also believed that the structure and efficacy of influenza viruses may be reduced in warm environments.

3. Seasonal variations in the host immune system

As is to be expected, equally 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 we ourselves also change both internally and externally depending on the time of year..

Here we move into much more intricate terrain, as the effectiveness of the host immune system is completely multifactorial and mediated by complex processes that we cannot cover in a few lines. For example, during the 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 sunlightThis results in a decrease of vitamin D levels in the host's body, which can weaken the effectiveness of the host's immune system. In relation to this issue, other studies also explore that food availability in certain regions during the winter is lower, which would result in 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, many factors, and for every study that shows a correlation between one of them and the presence of influenza, it is likely that another will not find clear evidence.

Conclusions

As we have seen in these lines, to the question "why does the flu appear in winter?" we cannot give a clear answer that applies in 100% of the cases. For example, in certain climates, the incidence of influenza shows epidemiological peaks during the rainy season, which does not make sense if we take into account that relative humidity has been shown to decrease the efficiency of virus transmission.

So what is more important, the survival of the virus, the behavior of humans or the immune system of the host? Unfortunately, we can't give you an answer yet. It is possible that a synergistic effect occurs between all the variations we have named for you, which would promote the occurrence of seasonal influenza epidemiological peaks.

Bibliographic references:

• Cox, N. (2014). Influenza seasonality: timing and formulation of vaccines.
• Influenza (Seasonal) World Health Organization (WHO). Retrieved August 16 from https://www.who.int/es/news-room/fact-sheets/detail/influenza-(seasonal).
• Influenza, Centers for Disease Control and Prevention (CDC). Recogido a 16 de agosto en https://espanol.cdc.gov/flu/index.htm
• Tamerius, J., Nelson, M. I., Zhou, S. Z., Viboud, C., Miller, M. A., & Alonso, W. J. (2011). Global influenza seasonality: reconciling patterns across temperate and tropical regions. Environmental health perspectives, 119(4), 439-445.
• Tamerius, J. D., Shaman, J., Alonso, W. J., Bloom-Feshbach, K., Uejio, C. K., Comrie, A., & Viboud, C. (2013). Environmental predictors of seasonal influenza epidemics across temperate and tropical climates. PLoS Pathog, 9(3), e1003194.