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Dry Air May Spur Flu Outbreaks

Researchers have long puzzled over why flu becomes so much more active in winter. A new study reveals that dry air is one likely culprit.

Scientists have proposed different explanations for why influenza makes more people sick in temperate regions during winter. One idea is that people simply spend more time indoors together because it's colder, giving the virus more opportunity to spread.

Another idea is that environmental factors affect the survival and transmission of the virus. For example, laboratory studies have found that higher temperatures affect the flu virus’s coat. That could potentially explain why flu doesn't spread during summer, but temperatures indoors, where most Americans spend the bulk of their time, are often tightly controlled.

Relative humidity is another suspect, but the data haven't established a strong link between relative humidity and flu outbreaks. Relative humidity, which is what you hear in weather reports, isn't the actual amount of water vapor in the air. Rather, given the current temperature, it tells you how close the air is to the point at which a cloud would start to form.

Dr. Jeffrey Shaman of Oregon State University wondered if absolute humidity—a measure of how much water vapor is in the air—could account for flu outbreaks. Last year, he reexamined laboratory data and found that absolute humidity could account for the airborne survival and transmission of the virus.

In the new study, Shaman and collaborators at several institutions, including NIH’s Fogarty International Center (FIC), compared death rates attributed to influenza over 31 years to absolute humidity readings nationwide. The researchers used a mathematical model of the influenza transmission cycle that incorporated Shaman's previous findings of how absolute humidity affects the survival and transmission of the virus. The study was funded by NIH's National Institute of General Medical Sciences (NIGMS), the Bill and Melinda Gates Foundation and others.

In PLoS Biology on February 23, 2010, the researchers reported that there were often significant drops in absolute humidity in the weeks prior to a flu outbreak. "This dry period is not a requirement for triggering an influenza outbreak, but it was present in 55-60% of the outbreaks we analyzed, so it appears to increase the likelihood of an outbreak," Shaman says. "The virus response is almost immediate; transmission and survival rates increase and about 10 days later, the observed influenza mortality rates follow."

This discovery might be used in the future to help predict when outbreaks will occur. It also has implications for treating influenza outbreaks. For example, hospitals may pay more attention to controlling humidity levels.

"Obviously there are tradeoffs because influenza is not the only pathogen out there," Shaman says. "There are pathogenic molds that flourish in higher humidity. But if the immediate concern is an outbreak of influenza, it may be worthwhile to raise humidity levels."

—by Harrison Wein, Ph.D.