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Idahome
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[font=proxima-nova, "Libre Franklin", "Helvetica Neue", helvetica, arial, sans-serif]Factor 1: The environment.
In the winter, the outdoor air is colder, and the air is dryer usually both indoors and out. For influenza, it has been elegantly shown in the lab that absolute humidity — the quantity of water vapor in the air — strongly affects flu transmission, with drier conditions being more favorable. Subsequently it has been shown that epidemiological patterns are consistent with this lab data in the US and in Vietnam, among other study sites. Notably the Vietnam study looked at influenza-like illness, without distinguishing influenza from other types of pathogens. This hints that similar mechanisms may be at work for other respiratory viruses, but to my knowledge are no specific studies of the role of humidity for coronaviruses or other respiratory viruses besides flu. Also important: there may be some very humid conditions that also favor flu transmission, especially relevant in the tropics. Still it is safe to say that in temperate countries, dry cold air = favorable conditions for flu transmission. For coronaviruses, the relevance of this factor is unknown. A recent preprint on which I collaborated suggests that transmission is possible in many different climates, and points out that Singapore, for example, which lies nearly on the equator, has had significant transmission. This is one piece of evidence, but as my colleague Dr. Eli Perencevich has pointed out there are many differences between Singapore in February and a temperate zone in summer — different day length, ultraviolet radiation, and other factors that may be important for coronavirus – we simply don't know.
[font=proxima-nova, "Libre Franklin", "Helvetica Neue", helvetica, arial, sans-serif]
[font=proxima-nova, "Libre Franklin", "Helvetica Neue", helvetica, arial, sans-serif]Factor 2: Human behavior.
In the winter humans spend more time indoors with less ventilation and less personal space than outdoors in the summer. In particular, schools are a site of much infectious disease transmission. School terms have been strongly identified as periods of higher transmission for respiratory viruses including those causing chicken pox, measles, and flu (here and here). The 2009 pandemic flu in the United States was very much decreased during the summer, and then came back rapidly in September.
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I build custom RBV, specializing in drivetrain conversions, wiring, suspension and complete custom trucks