Seasonal variability in the occurrence of respiratory viruses in temperate regions is well known, but the exact causes of, for example, increased wintertime influenza infection rates are not fully understood. A 4-year study in Gothenburg, Sweden, has suggested that one possible trigger for the onset of winter influenza epidemics may be a sudden drop in temperature leading to colder, drier air (in terms of absolute humidity). This in turn means that airborne particles containing the virus are typically smaller because of increased evaporation, hence can stay airborne for longer, facilitating increased transmission of disease. Influenza epidemic onsets were linked to the first time during the winter season that temperatures fell below 0°C and water vapour pressure below 4 hPa with a lag time of about 1 week, and influenza incidence continued rising following this even when temperatures increased. However, rhinovirus (often associated with common colds) did not show the same association with temperature and vapour pressure, and the authors speculated that their airborne transmission is mainly through larger droplets, whose behaviour is less affected by these meteorological parameters.
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