Ever wonder why you roll up your shirt for the flu shot about the same time you're adding a few layers on top of that shirt? The spread of influenza (commonly known as the flu) increases during winter months, but experts have had only guesses as to why. Some suggested it might be because people spend more time indoors in close proximity during the colder weather, or because people fleeing the cold also had less exposure to sunlight, which in turn was having a chemical effect on the virus itself or somehow weakened human immune response.

The spread of the flu is less about human behavior and more about weather.

Recent research from the Oregon State University suggests that the spread of the flu is less about human behavior and more about weather.

The Oregon State University researchers, from the College of Oceanic and Atmospheric Sciences and led by Jeffrey Shaman, looked at a 2007 study done by researchers at Mt. Sinai School of Medicine in New York. That particular study investigated the effects of temperature and relative humidity on transmission of influenza. They used influenza-infected guinea pigs in climate-controlled chambers in order to find what combination of temperature and relative humidity would be the most favorable for virus transmission.

The 2007 study found that infections increased when conditions were colder and drier, but the relationship between humidity and transmission seemed relatively weak. However, the 2007 research used the measure of relative humidity, which varies with temperature changes. The new study, published online in the February 9, 2009 issue of the Proceedings of the National Academy of Science, looked at the data using absolute humidity (the amount of water in the air, regardless of changes in temperature) as the measure. They found a much stronger link between low humidity and both flu virus survival and flu virus transmission.