# 2779
One of the longstanding mysteries about influenza is: Why is seasonal influenza predominantly a disease of winter?
Theories have ranged from the idea that the virus simply likes lower temperatures, to - during the winter - people are inside and clustered together more, and more likely to spread a respiratory illness.
In October of 2007, a study appeared in PLoS Pathogens entitled Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature by Anice C. Lowen, Samira Mubareka, John Steel, and Peter Palese.
I covered this story in a blog called Cold And Dry Statistics.
Using a guinea pig as a model host, they showed that airborne spread of the influenza virus was at least partially dependent upon both ambient relative humidity and temperature.
The link was significant, but not overwhelming.
Well, fast forward a little more than a year, and researchers Jeffrey Shaman and Melvin Kohn have re-analyzed the data, and found an even stronger correlation between the AH (Absolute Humidity) and the survival, and transmission of the virus.
Here is the abstract to this new research (reparagraphed for readability).
Absolute humidity modulates influenza survival, transmission, and seasonality
+Author Affiliations
- aCollege of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331; and
- bPublic Health Division, Oregon Department of Health Services, 800 NE Oregon, Suite 772, Portland, OR 97232
Edited by Burton H. Singer, Princeton University, Princeton, NJ, and approved January 7, 2009 (received for review July 16, 2008)
Abstract
Influenza A incidence peaks during winter in temperate regions. The basis for this pronounced seasonality is not understood, nor is it well documented how influenza A transmission principally occurs.
Previous studies indicate that relative humidity (RH) affects both influenza virus transmission (IVT) and influenza virus survival (IVS). Here, we reanalyze these data to explore the effects of absolute humidity on IVT and IVS.
We find that absolute humidity (AH) constrains both transmission efficiency and IVS much more significantly than RH. In the studies presented, 50% of IVT variability and 90% of IVS variability are explained by AH, whereas, respectively, only 12% and 36% are explained by RH. In temperate regions, both outdoor and indoor AH possess a strong seasonal cycle that minimizes in winter.
This seasonal cycle is consistent with a wintertime increase in IVS and IVT and may explain the seasonality of influenza. Thus, differences in AH provide a single, coherent, more physically sound explanation for the observed variability of IVS, IVT and influenza seasonality in temperate regions. This hypothesis can be further tested through future, additional laboratory, epidemiological and modeling studies.
During the summer, the ambient air often contains 4 times as much water as it does on a cool dry winter's day. This apparently is a significant factor in the spread of influenza viruses.
Interestingly, the Chinese have long boiled vinegar in their homes to ward off respiratory ailments, such as influenza. The noxious odor was supposed to `purify' the air inside the home, and newspapers still recommend this practice during their influenza season.
It may well be that the `active ingredient' is simply the water vapor being released, raising the absolute humidity in their homes, and thus lowering the survivability of the virus.
