PNAS | March 3, 2009 | vol. 106 | no. 9 | 3243–3248
Jeffrey Shamanaa and Melvin Kohnb
College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331;
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)
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.