Humidity affects influenza virus outbreaks

Washington, Feb 23 (ANI): Dry winters are linked to seasonal outbreaks of influenza, suggests a new study.

The study, which has appeared in PLoS Biology, also discovered that the onset of outbreaks might be encouraged by unusually dry weather conditions, at least in temperate regions.

For a long time scientists have suspected a link between humidity and seasonal (epidemic) flu outbreaks, but most of the previous studies focused on relative humidity - the ratio of water vapour content in the air to the saturating level, which varies with temperature. Absolute humidity quantifies the actual amount of water in the air, irrespective of temperature. Though somewhat counter-intuitive, absolute humidity is much higher in the summer.

Jeffrey Shaman, an Oregon State University atmospheric scientist and lead author said: "In some areas of the country, a typical summer day can have four times as much water vapor as a typical winter day - a difference that exists both indoors and outdoors."

For the study, the scientists used 31 years of observed absolute humidity conditions to drive a mathematical model of influenza and found that the model simulations reproduced the observed seasonal cycle of influenza throughout the US.

They first examined influenza in New York, Washington, Illinois, Arizona and Florida, and discovered that the absolute humidity conditions in those states all produced model-simulated seasonal outbreaks of influenza that correlated well with the observed seasonal cycle of influenza within each state.

Shaman and colleagues then extended their model to the rest of the continental U.S. and were able to reproduce the seasonal cycle of influenza elsewhere. They also discovered that the start of many influenza outbreaks during the winter was directly preceded by a period of weather that was drier than usual.

Shaman said: "This dry period is not a requirement for triggering an influenza outbreak, but it was present in 55 to 60 percent of the outbreaks we analyzed so it appears to increase the likelihood of an outbreak.

"The virus response is almost immediate; transmission and survival rates increase and about 10 days later, the observed influenza mortality rates follow." (ANI)