Atishoo!: Cool winter temperatures and low humidity may influence how rapidly colds and flu can spread.

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SYDNEY: It's not just an old wive's tale. Stay out in the cold, dry air and you're more likely to get flu, says a new study that explains why the malady spreads in winter.

There's no doubt that winter is the flu season; the virus spreads almost exclusively in the winter months in both northern and southern hemispheres.

But the reason for the distinct seasonal expression has commonly been attributed to people staying indoors where their sneezing and coughing can more rapidly spread in the confined spaces.

Spread dynamics

U.S. academics now confirm in the journal Public Library of Science Pathogens that temperature and humidity also influence how quickly the influenza virus spreads. The study may have implications for understanding the spread of cold viruses too.

"Low relative humidity and low temperature, conditions prevalent during the winter months… are conducive to the spread of influenza," said co-author Anice Lowen of Mount Sinai School of Medicine in New York City. "This is important because it supports the hypothesis that changes in the weather contribute to the seasonality of influenza."

To make the connection, Lowen and her co-workers housed infected and uninfected guinea pigs together in cages of differing temperature and relative humidity. At 30°C influenza was not transmitted at all, at 20°C it spread slowly, but at 5°C the virus spread much more rapidly. The researchers also found that at 80 per cent humidity, the virus didn't spread at all, but at lower humidity – around 20 to 35 per cent – the virus spread rapidly.

Temperature seemed to be more important than humidity, though, as the virus spread more easily in humid conditions when the room temperature was 5°C than when it was 20°C.

Decreased defences

To rule the chance that temperature was affecting the guinea pigs' immunity, the scientists tracked immune responses of the animals.

"Temperature may also affect the animal host, for example, by decreasing the effectiveness of primary defences against infection," Lowen said. That might include mucous becoming more viscous as cold air hits the respiratory passages. "We tested whether animals in the cold suffered immunosuppression and found that they did not," she said.

The researchers argue that temperature and humidity affect the stability of virus particles in the air, suggesting that turning up the heat in winter might have preventative benefits.

"Our results do suggest that increasing the heat and/or humidity during the winter could mitigate the spread of flu," she said. "This might be best applied in institutional settings where influenza is known to spread efficiently, like homes for the elderly and schools."

"It is clearly shown that cold and dry conditions favour transmission between animals," commented virologist Chris Burrell of the University of Adelaide in Australia. However, the study does not provide clear answers as to the mechanisms by which this spread takes place, he said, adding that that cold, dry temperatures could just be contributing factors to the spread of flu.