The coldest months of the year correspond with the flu season in North America.  In the Southern Hemisphere, cold months like June, July and August also bring flu outbreaks.  If you have wondered why there is a peak in respiratory infections in winter, you’re not alone.  The seasonal increase is thought to be the result of several combined factors, listed below:

  • Influenza A virus is more transmissible in chilly, dry air, as shown recently by Lowen, et. al.
  • Reduced sunlight in winter decreases Vitamin D production, which decreases immune response.
  • There is less ultraviolet light and ozone gas in the air in winter, which may prolong survival of influenza virus in outdoor air.
  • Cold air may suppress the local immune system in the upper respiratory tract, increasing infection risk.
  • School starts in the fall, then holiday gatherings take place in November, December, and January, bringing sick people into contact with healthy people.
  • Colder temperatures enhance survival of influenza virus on surfaces.

Influenza virus is mainly spread by air, so using air filters and covering coughs and sneezes will help to reduce spread of the disease. 


Want to check out real-time flu activity in your area?  Check out Google Flu Trends, which is based on internet searches for terms related to flu infection and updated regularly to improve the model’s fit with outbreak data.