Researchers from Sweden used 20,000 virus samples and weather statistics to determine that a cold week with an average temperature below freezing typically precedes the start of that season's flu epidemic.
The research team looked at three seasons' worth of virus samples and compared the incidence of respiratory viruses over time with weather data. They determined flu outbreaks are activated roughly one week after the first really cold week with low humidity.
"We believe that this sudden drop in temperature contributes to 'kickstart' the epidemic," said Nicklas Sundell, a researcher at Sahlgrenska Academy in Sweden and an infectious disease specialist at Sahlgrenska University Hospital. "Once the epidemic has started, it continues even if temperatures rise. Once people are sick and contagious, many more may become infected."
Not only do weather conditions contribute to the spread of influenza viruses, they are also affect the spread of other viruses that cause respiratory infections.
"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," Mr. Sundell said.
The study is published in the Journal of Clinical Virology.