Cold Weather May ‘Kickstart’ Flu Epidemics Each Year


If you have ever rolled your eyes at your grandmother when she warned you that being out in the cold will make you sick, it may be time to apologize. A recent study found how cold weather and the spread of viruses are linked.

Seasonal flu outbreaks first happen about a week after the winter’s first cold spell, according to the Swedish study published in the Journal of Clinical Virology.

Credit: Tazrian Khan/Flickr, CC BY 2.0

“According to our calculations, a cold week with an average temperature below zero degree Celsius precedes the start of the flu epidemic,” Nicklas Sundell, a researcher at Sahlgrenska Academy and infectious diseases specialist at Sahlgrenska University Hospital, said in a statement.

During the course of three years, researchers collected more than 20,000 nasal swabs from sick people seeking medical care in the Swedish city of Gothenburg. They analyzed the swabs for influenza A and other respiratory viruses, and then compared their findings with weather data from the Swedish Meteorological and Hydrological Institute.

The researchers found that the first very cold week of the year, where the humidity was low and temperatures were below freezing, seemed to help the spread of the flu.

“We believe that this sudden drop in temperature contributes to ‘kickstart’ the epidemic,” Sundell said. “Once the epidemic has started, it continues even if temperatures rise. Once people are sick and contagious, many more may become infected.”

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Cold weather, they found, helps airborne particles containing liquid and virus — like from a sneeze — spread easily and quickly. The dry air absorbs the moisture and shrinks the particle, helping it stay in the air longer and travel larger distances.

The study also discovered that other common respiratory infections exhibited similar patterns when exposed to dry, cold weather. However, other rhinoviruses like the common cold did not seem to be affected by the weather.

“Cold and dry weather and small aerosol particles are important prerequisites for the flu epidemic to take off,” Sundell said. “But cold weather isn’t the only contributing factor. The virus has to be present among the population and there have to be enough people susceptible to the infection.”

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Sundell hopes that his research can help doctors and medical professionals better prepare for flu season, as well as inform the public about those risks.

“If you can predict the start of the annual epidemics of the flu and other respiratory viruses, you can use this knowledge to promote campaigns for the flu vaccine and prepare emergency wards and hospital staff in advance for an increased number of patients seeking care,” Sundell said in a statement.

This year’s cold and flu season in the United States is on track to be worse than last, according the Centers for Disease Control and Prevention. In order to help prevent coming down with the flu, Sundell recommends vaccination as well as some common sense tips: “cough and sneeze into your elbow, and remember to wash your hands.”