Absolute Humidity as a Deterministic Factor Affecting Seasonal Influenza Epidemics in Japan

Influenza epidemics occur periodically during the winter season in temperate areas. Characteristic features of winter include low temperature and low humidity. Humidity is expressed in two different ways: absolute humidity (AH) defined as absolute amount of water in the air, and relative humidity (RH) defined as the relative proportion of water in the air in comparison to the maximum water vapor. There have been many arguments for RH as a determinant factor for influenza epidemics. On the other hand, we have been putting emphasis on AH on the basis of our epidemiological observations. In this context, a recent experimental and theoretical study by other investigators has shown that AH correlates with influenza survival, transmission, and seasonality. Accordingly, we collected meteorological and influenza epidemiological data from 46 prefectures in temperate Japan for 1991-1995 and 1999-2009, and analyzed 2,392 sets of weekly compiled data for each season year by using multiple linear regression analysis, in which the numbers of influenza cases were regarded as a function of AH and RH. We found that the standardized partial regression coefficient for AH was consistently stronger than that for RH with statistical significance. In addition, AH increased and decreased significantly at the time of the epidemic onset and subsidence in seven and twelve out of fourteen influenza seasons, respectively, whereas RH did so in none and two out of fourteen influenza seasons. Thus, we have substantiated our quarter-century-old assertion that AH strongly correlates with the onset and subsidence of influenza epidemics.