Dry Weather Boosts Odds of Flu Outbreaks

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TUESDAY, Feb. 23 (HealthDay News) -- Rain and snow may seem like perfect incubators for the flu, but new research suggests that low humidity and unusually dry skies might be responsible for increases in influenza, such as those that occur during winter months.

Previous research has suggested that humidity is connected to seasonal epidemics of flu, but studies have usually focused on relative humidity, as opposed to absolute humidity. Relative humidity, which varies with temperature, is the ratio of water vapor content in the air to the saturating level. Absolute humidity, which is the actual level of water in the air, does not depend on temperature, but often reaches lower levels in the winter than in the summer.

"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," Jeffrey Shaman, an Oregon State University atmospheric scientist and lead author of the new study, said in a news release from the Public Library of Science.

Shaman and his colleagues created a mathematical model of influenza and plugged 31 years of absolute humidity data into it. They found that influenza outbreaks in the winter often happened right after a period of unusually dry weather, according to their report published online Feb. 22 in PLoS Biology.

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

Irene Eckstrand, of the U.S. National Institute of General Medical Sciences program, explained in the news release: "The discovery of a link between influenza outbreaks and absolute humidity could have a major impact on the development of strategies for limiting the spread of infection. Understanding why outbreaks arise is an important first step toward containing or even preventing them, so it is essential for scientists to follow-up on this intriguing connection."

More information

The U.S. Centers for Disease Control and Prevention has more on the flu.