The predominant route of transmission of H1N1 in the tropics could be through contact; in the case of the temperate regions, it could be the aerosol route.

According to the hypothesis published in the PLoS Currents: Influenza website, the authors from the Mount Sinai School of Medicine, New York, note that aerosol transmission is acutely sensitive to relative humidity and temperature, while contact transmission is not.

By extending the hypothesis, they state that aerosol transmission in the temperate zone could be seen during winter season (November to March) and a year-round transmission in the tropics.

The authors, Anice Lowen and Peter Palese, had in 2007 used guinea pigs to study the route of transmission of seasonal influenza virus and the conditions favouring its transmission. They found that cold and dry conditions seen during winter in the temperate regions were most favourable for aerosol transmission. This form of transmission was blocked when the relative humidity was 80 per cent or when it got warmer than 30oC.

The experiments showed that contact route was neither affected by humidity nor by temperature. The authors hypothesise that transmission could be seen throughout the year in the tropics if the predominant route is contact.

However, the H1N1 outbreak occurred in April and spread rapidly in the temperate regions after the November-March peak period. Thus the emergence and spread are in conflict with the hypothesis.

The authors suggest four possible explanations for this: increased surveillance and case detection; transmission was possible despite hostile conditions as H1N1 is a unique viral; the lack of natural immunity in humans allowed the transmission even in hostile conditions and finally; the rapid spread was possible as transmission was through the contact route, and the absence of in-built immunity in humans facilitated this.

Of the four possibilities, the authors favour the last one — transmission through contact in the absence of immunity to H1N1. While ruling out the first scenario, they note that "the remaining hypotheses are all feasible and time and experimentation will tell which is correct".

If aerosol transmission is the predominant route of transmission in the temperate region, then "increased transmission in the coming winter months" can be expected.

As for the tropics, the H1N1 viral spread can be seen throughout the year.

The PLoS Currents: Influenza website is a beta version for rapid dissemination of any news on H1N1. It comes from the stable of the Public Library of Sciences (PLoS) journals.

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