The reason why cold weather can cause a flu epidemic has finally been revealed.
Each year, a national flu epidemic always follows a chilly dry week with average temperatures below 0C.
Now, new research suggests that cold dry weather makes for the perfect conditions as it helps the virus stay airborne for longer.
Scientists studied 20,000 virus samples taken from nasal swabs.
They then compared the incidence of respiratory viruses with weather data from the Swedish Meteorological and Hydrological Institute (SMHI).
The results showed flu outbreaks were activated one week after the first cold snap, with low ourdoor temperatures and low humiditiy.
Nicklas Sundell, a researcher at Sahlgrenska University Hospital, said: “According to our calculations, a cold week with an average temperature below zero degree Celsius precedes the start of the flu epidemic.”
The study suggests that aerosol particles containing virus and liquid are more able to spread in cold and dry weather.
But if the surrounding air is dry, it absorbs moisture and the aerosol particles shrink and can remain airborne.
Mr Sundell said: “We believe that this sudden drop in temperature contributes to “kickstart” the epidemic.
“Once the epidemic has started, it continues even if temperatures rise. Once people are sick and contagious, many more may become infected.”
The results suggested that a cold snap probably plays a bigger part in prompting a flu outbreak than indoor crowding during poor weather.
A number of other common viruses, including the RS-virus and coronavirus also spread faster in cold and dry conditions.
But other viruses such as rhinovirus, that are a common cause of cold, are independent of weather factors and is present all year around.
Mr Sundell said: “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.
“The recommendations are the same as previous years: vaccination of risk groups, cough and sneeze into your elbow, and remember to wash your hands.”