Flu Season Dependent On Weather And City Transportation Patterns
New research suggests the spread of flu in your town depends on both the weather and the way people move around.
The more people a city has — and the more organized their movements are — the longer its flu season is likely to last, according to new research from Oregon State University.
OSU College of Science population biologist Benjamin Dalziel analyzed flu data from more than 600 cities, paying attention to the relative humidity in each city and the way residents got to work. He wanted to know if residents regularly converge together, like in a subway system, or if were they more likely to drive to work by themselves.
What Dalziel found was that when there are lots of people, and transportation patterns draw them together, flu viruses are better able to find new hosts, even when climatic conditions aren’t at their most favorable.
“We found that as cities become larger and mobility patterns become more highly organized, climatic conditions play a relatively smaller role in influenza transmission,” Dalziel said.
Flu is transmitted by moisture droplets that sick people exhale. The droplets contain the flu virus, which is inhaled by people nearby. Dalziel calls it a "cloud of risk."
“As specific humidity decreases, the virus remains viable in the air for longer, effectively expanding that cloud,” Dalziel said. “However, if an infected person is right beside you, it matters less what the specific humidity is.”
Dalziel also found that in smaller cities, where transportation isn’t as coordinated, flu cases tend to be tightly grouped in a short period of time, during peak season, when climate conditions are best for transmission.
“One thing that distinguishes urban centers from small towns is the presence of localized pockets of high population density that are connected by organized movement patterns,” Dalziel said. “We found that makes a difference for how influenza spreads at different times of year.”
Dalziel and his team found that in metropolises, flu cases are more spread out through the winter months, including early and late in the season, when the weather is not optimal for transmission. By contrast, in smaller cities, more cases are tightly grouped in a short period of time, during peak season, when climate conditions are best for transmission.
The findings are published in the journal Science. They might be used to predict viral outbreak trends that sicken millions of people in the U.S. and kill tens of thousands each year.
“Forecasting and controlling influenza is important for public health,” Dalziel said.
Researchers were anxious to point out that their work does not show some cities are safer than others for flu, “Rather it shows relative differences in when the cases are likely to occur,” said Dalziel.
Researchers from the University of Cambridge, Princeton University, Pennsylvania State University and the National Institutes of Health collaborated on this study. It was funded by The Bill & Melinda Gates Foundation, the Department of Homeland Security and The National Institutes of Health. [Copyright 2018 Oregon Public Broadcasting]
