| Summary: | Understanding the transmission of influenza viruses is of prime importance for both vaccine design and vaccination
policies. A number of factors have been explored as potential drivers of human seasonal influenza including human
behaviour, dynamics of immunity driven by viral evolution (Smith et al. 2004), and climatic factors. Climatic factors
have, in particular, retained attention from researchers because of the modes of influenza transmission that impose
the virus to spend a substantial amount of time in the air. Thus, climatic variables have been hypothesised to influence
host susceptibility and viral survival and dispersal in the air. Because viral particles are carried in droplets in the air,
humidity has long been considered as a perfect candidate to explain influenza transmission, without much success
to prove it, until recently. In 2009, the re-examination of experimental data conducted on Guinea pigs (Lowen et al.
2007) showed a strong negative correlation between humidity and influenza transmission if absolute humidity (the
quantity of water in the air) was considered instead of the more common measure of relative humidity (Shaman
and Kohn 2009). This result was confirmed two years later with epidemiological data from the USA, showing that,
every year, influenza epidemics were triggered by a drop in absolute humidity (Shaman et al. 2010). These results
provided a reasonable understanding of the drivers of the influenza epidemiology, at least in temperate countries
where the epidemiology was highly seasonal. However, when looking at other countries in the world, it appeared that
the intensity of seasonality faded out the closer we got to the equator (Viboud et al. 2006). (Copied from article.)
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