Most acute viral infections have a clear seasonal variation in incidence. In general, respiratory virus infections are more prevalent in winter months, while infections with enteric viruses predominate in the summer. In temperate climates, influenza mainly occurs from November to March in the northern hemisphere. There have been a number of suggestions to explain this seasonal incidence, but the most compelling is supported by recent data using a guinea pig model of influenza. In this study, spread of influenza virus in aerosols was shown to be dependent upon temperature and relative humidity.
To study transmission of influenza, the authors housed infected and uninfected guinea pigs in an environmental chamber. They found that transmission of infection was most efficient when the humidity was 20-35%; it was blocked at 80% humidity. Furthermore, transmission was more likely to occur at 5Â°C than at 20Â°C. The authors concluded that conditions found during winter, low temperature and humidity, favor spread of the infection.
Why is influenza transmission dependent on low humidity? One factor is the stability of the virion: it has been shown thatÂ viral stability is maximal at low humidity (20%â€“40%), minimal at intermediate humidity (50%), and high at elevated humidity (60%â€“80%). Another factor isÂ the respiratory droplet in which influenza viruses are transmitted. At low humidity, exhaled aerosols would rapidly lose water, leading to the formation of droplet nuclei. These particles are less than Â 5 Î¼m in diameter and remain airborne for long periods, facilitating virus transmission. At high humidity, respiratory droplets absorb water, becoming larger and falling to the ground more rapidly than droplet nuclei. The authorsÂ propose “that at high RH (80%) exhaled respiratory droplets settle too rapidly to contribute to influenza virus spread.”
Curiously, influenza aerosol transmission was blocked at 30Â°C. This observation is at odds with the fact that influenza occurs all year round in tropical climates. The authors show in a subsequent paper that contact transmission still occurs at this high temperature. Therefore it is possible that contact or very short-range spread is the predominant mode of influenza transmission in the tropics.
You could try keeping your home warm and moist in the winter to minimize influenza transmission. However, sooner or later you will encounter a cool, dry environment and all your precautions will be tossed to the wind.