Published: Feb 2010 |  PLoS Biol 8(2): e1000316. doi:10.1371/journal.pbio.1000316

Absolute humidity and the seasonal onset of influenza in the continental United States

Shaman J et al.


Influenza A incidence peak during winter in temperate regions. The cause of seasonality in influenza transmission is of interest for public health and basic science but not well understood.

Studies indicate that relative humidity (RH) affects both influenza virus transmission (IVT) and influenza virus survival (IVS). The authors reanalyzed this data to explore the relationship between outdoor absolute humidity (AH) and IVT and IVS. They note that absolute humidity (AH) affects both transmission efficiency and IVS more significantly than RH.

“In temperate regions, both outdoor and indoor AH possess a strong seasonal cycle that minimizes in winter. This seasonal cycle is consistent with a wintertime increase in IVS and IVT and may explain the seasonality of influenza”.

By considering the daily negative deviation of AH from the multiyear daily average in US States, the authors modeled the excess “Pneumonia and Influenza” rate (P&I) by using a classical SIRS model (Susceptible - Infected - Resistant - Re-Susceptible). Negative deviations could typically be observed in the 4 weeks prior to the onset of influenza epidemics. The simulation of yearly curves for mean daily excess of P&I mortality fitted well with the observed statistical data.

Authors suggest that with negative deviation data of outdoor AH, short-term probabilistic forecasts of epidemic influenza could be developed.

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by Walter Hugentobler

This study shows that low outdoor absolute humidity appears to be a reliable forecasting parameter for the onset of seasonal influenza epidemics.

However, as direct or indirect contact, physical or by “air sharing”, is an absolute prerequisite for any microbial transmission, and this largely occurs indoors, it would follow that it is indoor humidity rather than outdoor that would be the main causative factor in the spread of flu.

When outdoor air with low absolute humidity is brought indoors and heated, the indoor relative humidity drops.

There is nothing we can do about low outdoor humidity but we can affect the moisture levels in buildings through the use of hygienic humidification systems. In a world of buildings that have their internal humidity optimally maintained, flu epidemics would be very much contained, if not eliminated. The flu and common cold would be de-coupled from the seasonal highs and lows of the outdoor air.

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