Why does the flu appear in winter?

Influenza is an infectious disease caused by influenza A virus or influenza B virus, two RNA viruses of the Orthomyxoviridae family. This pathology exhibits a clear seasonal pattern, as it usually shows epidemiological peaks in late fall and winter in temperate climates.

But, Why does the flu appear in winter? Several studies attempt to answer this question which, although it does not appear to be the case, does not yet 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 response

    First, 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 the seasonal flu. These are:

    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 of the pandemics recorded with regard to influenza have been caused by these influenza viruses. These are the most aggressive pathogens of the types responsible for the pathology.

    However, influenza B viruses are less common and less aggressive than those named above. Their low mutation rate and unique ability to infect humans and seals (as opposed to type A ones, which have many more hosts) make these viral variants less likely to produce an epidemic. Currently circulating can be divided into two lines B / Yamagata and B / Victoria.

    Finally, we can also find influenzaviruses of type C and D, although they are very rare and their epidemiological importance is reserved for specific foci.

    So, as we have seen, the world of influenza is much larger than one might initially imagine on a purely microscopic level. Yet, there is much more to study when it comes to infectious 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 10-20% of the general population.. It will be said soon, but it translates into a fifth of all people on Earth presenting a photo of the flu as you read this.

    The population groups considered “at risk”, according to the WHO, are pregnant women, babies under 59 months and patients with chronic diseases or immunosuppression (as is the case with 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 next question: why does influenza appear in winter?

    The reason for seasonality

    It should be noted, first of all, that the reason for the seasonality of the flu is not entirely clear. From now on, we will evolve in hypotheses, certainly supported by scientific studies, but which can in no way be considered as absolute realities.

    In addition, as many professionals say, “sometimes science is based more on asking the right questions than on detecting unmistakable realities.”

    Here are 3 possible explanations for the seasonality of the flu.

    1. Seasonal variations in contact

    It is clear that the behavior of hosts (in this case humans) must play a critical role in the spread of disease in any epidemiological process.

    Several studies hypothesize that the fact that people crowd us more in public spaces during the winter can help the spread of the flu. These are based on the fact that, for example, in the United States, any supervised person spends an average of 1 to 2 hours more indoors during episodes of cold weather.

    To provide further evidence in light of the above, other sources argue that the transmission of influenza to children during the holidays is reduced by almost 30% compared to periods of schooling.

    Viruses are naturally transmitted by secretions of aqueous microparticles present in coughs and sneezes. The more people there are accumulated in a sealed space without ventilation, the easier it will be to inhale the polluted emissions.

    While all of this evidence seems to show us that it is “off the beaten track” that variations in human behavior during winter are responsible for peaks in winter flu, there is no empirical evidence linking the two factors. completely reliably.

    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 you survive in the outside environment, the more likely a healthy person will be infected, right?

    According to this reasoning, 4 out of 6 medical studies have shown that the survival of influenza viruses increases the lower the relative humidity of the environment. It might be possible because at higher humidity, the virus is more likely to adhere to water vapor particles in the air, Which would cause it to precipitate early, rather than being able to infect another host.

    The relationship between temperature and viral efficacy is not as clear or demonstrated, but preliminary studies in laboratory animals have shown that at 30 degrees room temperature, infection is predicted in its entirety. This is why it is also believed that the structure and effectiveness of influenza viruses could be reduced in hot environments.

      3. Seasonal variations in the host’s immune system

      As expected, the host’s defense barriers against an epidemiological process are just as important as the tools for virus transmission. Therefore, we cannot forget that we ourselves have changed both internally and externally depending on the time of year.

      Here we are operating on a much more complex ground, as the efficiency of the host immune system is completely multifactorial and mediated by complex processes which we cannot cover in a few lines. For example, during the winter, the combined effect of cold and dryness results in a loss of moisture from the nasal mucosa, which could facilitate the entry and multiplication of viruses.

      Another factor could be the lack of sunlightAs this results in decreased levels of vitamin D in the host’s body, which can weaken the effectiveness of your immune system. Related to this question, other studies also explore that food availability in some areas during 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 factors, and for every study that shows a correlation between one of these and the presence of the flu, it’s likely that another won’t. will not find clear evidence.


      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 100% of the time. For example, in some climates, the incidence of influenza shows epidemiological peaks during the rainy season, which makes no sense considering that relative humidity decreases the efficiency of virus transmission.

      So what is more important, the survival of the virus, human behavior or the host’s immune system? Unfortunately, we still cannot give you an answer. It is possible that a synergistic effect occurs between all the variations that we have named for you, which would favor the appearance of influenza epidemiological peaks according to the seasons.

      Bibliographical references:

      • Cox, N. (2014). Seasonality of influenza: 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)
      • Influeza, Centers for Disease Control and Prevention (CDC). Retrieved August 16, from https://espanol.cdc.gov/flu/index.htm
      • Tamerius, J., Nelson, MI, Zhou, SZ, Viboud, C., Miller, MA and Alonso, WJ (2011). Global seasonality of influenza: models of reconciliation between temperate and tropical regions. Environmental Health Perspectives, 119 (4), 439-445.
      • Tamerius, JD, Shaman, J., Alonso, WJ, Bloom-Feshbach, K., Uejio, CK, Comrie, A. and Viboud, C. (2013). Environmental forecasts of seasonal influenza epidemics in temperate and tropical climates. PLoS Pathog, 9 (3), e1003194.

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