科学美国人60秒:U.S. Flu Spread Counts On Southern Cold Snaps

If you’ve got the flu, your focus is on getting better—not on how you caught it. But from a public health standpoint, tracking how flu spreads can help keep the virus contained.

In the past, models predicting the path of epidemics have focused on travel by plane—in some cases combining data on population density with airport locations. And when studies showed that influenza transmission is modulated by humidity, scientists injected information on climate into the mix.

Now, a new study combines data on a variety of factors, from doctors’ visits and vaccination coverage to weather patterns and the movement of individuals as recorded by Twitter. The finding: in the U.S., influenza typically arises in the warm, humid conditions of the south and spreads quickly thanks to the high degree of social connectivity in the region. The finding is in the journal eLife. [Ishanu Chattopadhyay et al., Conjunction of factors triggering waves of seasonal influenza]

The researchers started by poring over health care records from more than 40 million families, looking for reports of flulike symptoms. The analysis covered nine season’s worth of data, from 2003 to 2011. And it pointed toward outbreaks starting near the Gulf of Mexico or the southern Atlantic, a surge that seemed to coincide with the southward migration of ducks.

“We did our first analysis and it did look like ducks could be possible carriers of the virus, starting spark of the influenza epidemic.”

Andrey Rzhetsky of the University of Chicago, the study’s senior author. He says they wrote up their “duck hypothesis” and submitted the paper for review.

“Reviewers, so to speak, strongly encouraged us to include additional factors into the model, specifically climate variables, temperature, wind speeds, solar radiation, humidity.”

And when they did…

“Lo and behold, duck hypothesis collapsed. Ducks as predictor were not important anymore and climate took the first place.”

Based on the data they collected, the researchers liken the spread of flu to a wildfire. The spark that ignites the epidemic is provided when a blast of colder weather strikes an otherwise warm, humid, urban environment. That chill allows the virus to remain viable in water droplets and perhaps forces people indoors into close quarters.

That’s where Southern hospitality comes in. Folks in the south are more highly socially connected than elsewhere in the country. So friends, neighbors and community members have plenty of opportunity to pass the virus to one another face to face.

Finally, driving from county to county or traveling by plane, allows the flu to spread like wind carries a fire. So ya’ll come back now—but first make sure you’re no longer contagious.

—Karen Hopkin