The impression that the flu appears during winter is very common but does it have any real basis?

ResearchBlogging.orgIn a broad way and with many exceptions, infectious diseases may be divided into acute and chronic. The acute infection occurs when the virus quickly infects the host, causes symptoms, is or is not transmitted and the disease is contained. It is the case of influenza, measles and dengue. In contrast, there are chronic infections such as hepatitis C, HIV and Tuberculosis, which cause long-term diseases and lighter symptoms at the first time.

Not rare, the virus that causes acute infection has a seasonal cycle, that is, with well marked waves of epidemics. The cycles can be regulated by various factors. Seasons of the year, immunity of the susceptible population, life cycle of the vector (as the case of dengue, which depends on the Aedes aegipty mosquito that reproduces during the raining season in order to be transmitted) are some of the influencing factors. [1]

Porcentagem de casos de gripe por semana. Países mais ao norte a ao sul possuem picos bem concentrados de casos. Fonte: referência 4.

Percentage of flu cases per week. Countries closer to the north and the south such as USA and Argentina have well concentrated cases. Source: reference 4.

In the case of Influenza, the seasonality is well characterized and latitude-dependent. Countries of temperate climate, closer to the North and the South have well defined flu seasons during winter, which corresponds to the months of December, January and February in the North and June, July and August in the south. Tropical countries, however, maintain a relatively constant number of cases during the year. The known possible reasons for this are many, but still far from being established.

We can see in the graph that countries in the more extreme latitudes, the incidence of cases is concentrated during some weeks of the year, the common flu season that lasts from 5 to 10 weeks. This is why the vaccination campaigns have to occur about one month in advance. Thus, there is enough time to produce immunity before the arrival of winter.

Mechanisms associated to the virus and host have been proposed to explain this cycle. From the human part, various agglomerations in closed environments during winter, coexistence of children at school and even fluctuations in our metabolism and immune response, such as decline of melatonin and vitamin D during winter have been considered possible explanations. [2]

More recently, studies focused on the properties of the virus raised promising results. In a recent reanalysis of the data presented by several articles, it was observed that the pressure of air vapor, that is, the absolute quantity of water dissolved in air, has a negative effect on Influenza. The greater the absolute humidity, the lesser is the transmission and the viability of virus in the air. – Relative humidity is the quantity of water dissolved in air in respect to how it behaves before precipitation (such as rain), the hotter it is, the more water can be dissolved; absolute humidity is the total quantity of water dissolved in air, regardless of how close it is to precipitating.

That is, during winter, the air is colder and there is less water dissolved in it. Therefore, the virus survives (continues to be infectious) in the air for a longer period of time and is transmitted more efficiently. During summer, the temperatures increase, more water is dissolved in the air and the virus is viable for a lesser period of time, making the cases of flu to be concentrated in winter. These results suggest that in closed places, such as daycare centers and schools, air humidifiers can be a good way of preventing the transmission of flu. [3]

The fact that the virus is best transmitted at a low vapor pressure can be related to the formation of aerosols and their remaining in the air for longer periods of time. Therefore, the doubt remains which would be the transmission way in countries with tropical climate that can maintain the virus in circulation along all the year.

The idea in this case is that, probably, in tropical countries, the transmission occurs especially through ways that involve contact, the contact with sick people and with contaminated surfaces, which came into contact with saliva or mucus containing the virus.

This will help explain the transmission of Influenza A (H1N1) in countries that had summer season from June to August. As the majority of the population do not have any prior immunity against this virus, it replicates more efficiently and also as a result of easier transmission by contact, once the conventional H1N1 and the H3N2 were not detected circulating during the same period of time. [4]

The seasonal cycles regulate the production of vaccines and the circulation of new variants of the virus, and are one of the main points still unanswered about the flu. Despite all the understanding that we have about the flu, one of the main points about the epidemics that still remains unknown and shows how much we still have to study and understand about the Influenza.

[1] Leggiadro, R. (2001). Seasonal Variation of Host Susceptibility and Cycles of Certain Infectious Diseases The Pediatric Infectious Disease Journal, 20 (10) DOI: 10.1097/00006454-200110000-00027
[2] Lipsitch, M., & Viboud, C. (2009). Influenza seasonality: Lifting the fog Proceedings of the National Academy of Sciences, 106 (10), 3645-3646 DOI: 10.1073/pnas.0900933106
[3] Shaman, J., & Kohn, M. (2009). Absolute humidity modulates influenza survival, transmission, and seasonality Proceedings of the National Academy of Sciences, 106 (9), 3243-3248 DOI: 10.1073/pnas.0806852106
[4] Lowen, Anice; Palese, Peter. (2009). Transmission of influenza virus in temperate zones is predominantly by aerosol, in the tropics by contact: A hypothesis. PLoS Currents Influenza:RRN1002.