Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature

TLDR

The seasonal epidemiology of influenza is well characterized, but the reasons for its predominant wintertime spread remain unclear. The study aims to provide direct experimental evidence that weather conditions influence influenza dynamics and to investigate the mechanism of prolonged viral growth by measuring innate immune mediators in the upper respiratory tract. The authors measured expression levels of innate immune mediators in the upper respiratory tract to investigate the mechanism of prolonged viral growth. In guinea pigs, aerosol transmission of influenza is increased under cold (5 °C) and dry (≤20 % RH) conditions, with prolonged shedding at 5 °C driving higher spread, while innate immune responses remain similar across temperatures, suggesting humidity and temperature act mainly on the virus particle.

Abstract

Using the guinea pig as a model host, we show that aerosol spread of influenza virus is dependent upon both ambient relative humidity and temperature. Twenty experiments performed at relative humidities from 20% to 80% and 5 °C, 20 °C, or 30 °C indicated that both cold and dry conditions favor transmission. The relationship between transmission via aerosols and relative humidity at 20 °C is similar to that previously reported for the stability of influenza viruses (except at high relative humidity, 80%), implying that the effects of humidity act largely at the level of the virus particle. For infected guinea pigs housed at 5 °C, the duration of peak shedding was approximately 40 h longer than that of animals housed at 20 °C; this increased shedding likely accounts for the enhanced transmission seen at 5 °C. To investigate the mechanism permitting prolonged viral growth, expression levels in the upper respiratory tract of several innate immune mediators were determined. Innate responses proved to be comparable between animals housed at 5 °C and 20 °C, suggesting that cold temperature (5 °C) does not impair the innate immune response in this system. Although the seasonal epidemiology of influenza is well characterized, the underlying reasons for predominant wintertime spread are not clear. We provide direct, experimental evidence to support the role of weather conditions in the dynamics of influenza and thereby address a long-standing question fundamental to the understanding of influenza epidemiology and evolution.