A nurse injects a flu vaccine at the Whitman-Walker Health Clinic in Washington, DC on January 10, 2013. The Center for Disease Control and Prevention has said that this year's flu season is expected to be one of the worst the country has seen in 10 years. (EVA HAMBACH/AFP/GETTY IMAGES)

Flu has reached widespread levels in 47 states. Does cold, dry weather lead to flu outbreaks? The cold not so much. Dry weather may play a role, although it’s a controversial one.

Some studies have found cold weather may help flu linger in the air, but researchers tend to shoot down the idea it causes flu to spread. They stress flu is mostly transmitted by children moving indoors during the cold months when germs are exchanged rather than by the cold itself.

Dr. Jon Abramson, a specialist in pediatric infectious diseases at Wake Forest Baptist Health, told ABC News the flu season better correlates with the timing of the school year than with temperature.

Rather than cold weather, it may be dry weather which can help predict flu outbreaks.

In 2010, Jeffrey Shaman and colleagues published research showing flu outbreaks often occurred immediately following a dry spell.

“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 told Health magazine. “The virus response is almost immediate; transmission and survival rates increase and about 10 days later, the observed influenza mortality rates follow.”

Shaman, who works at Columbia University’s Mailman School of Public Health, relies - in part - on the dry weather and flu relationship in a state of the art model he developed which he claims can predict flu outbreaks more than 7 weeks ahead of time. (I wrote about this in late November, see: Study: weather forecasting science can help predict flu outbreaks).

Scott Sheridan, a professor of climatology at Kent State University, believes there is promise in Shaman’s approach.

“I’ve seen several papers that show a pretty convincing relationship between the dew point [a measure of humidity] drop and influenza, and so would certainly believe this to be a plausible relationship to exploit,” Sheridan said.

But Adam Kalkstein, a climatology professor at the U.S. Military Academy, who is studying weather and flu linkages, questioned relying on a humidity and flu link as a key basis for making flu predictions.

“If [absolute humidity] was the sole factor in determining influenza, the rates and timing of the flu would vary tremendously across the country... this is simply not occurring,” Kalkstein said.

Similarly, Larry Kalkstein (incidentally Adam Kalkstein’s father), a researcher professor at the University of Miami and an expert on weather, climate, and health, stressed the importance of non-weather factors in flu transmission.

“It has been difficult to develop an influenza/weather approach that has any degree of accuracy because of the many intervening variables that are not related to meteorology,” Kalkstein said

But Kristie Ebi, an epidemiologist who specializes in climate, said considering environmental factors like weather and human contact together may hold the key to understanding flu transmission: “Person-to-person transmission does not explain the start of the influenza epidemics, although it does explain transmission once an epidemic starts. Over the past couple of years, there have been significant advances in understanding the environmental determinants of influenza.”