Study: Humidity may help explain seasonality of flu virus

BLACKSBURG, Va. — Humidity may be the common denominator to help explain why flu viruses are most prevalent during the winter months in temperate climates like the United States, and most prevalent during the rainy season in many tropical regions close to the equator, according to a study released Tuesday by Virginia Tech researchers.

Possible explanations for the seasonality of the flu have been investigated in the past, such as the return of kids to school, people spending more time indoors in the winter and lower light levels that affect the immune system, but there has been no consensus among that research.

The Virginia Tech study presents for the first time the relationship between the influenza A virus viability in human mucus and humidity over a large range of relative humidities, from 17% to 100%. At low humidity, respiratory droplets evaporate completely and the virus survives well under dry conditions. But at moderate humidity, the droplets evaporate some, but not completely, leaving the virus exposed to higher levels of chemicals in the fluid and compromising the virus’ ability to infect cells.

Researchers found the viability of the flu A virus was highest when the relative humidity was either close to 100% or below 50%. “We added flu viruses to droplets of simulated respiratory fluid and to actual human mucus and then measured what fraction survived after exposure to low, medium, and high relative humidities,” stated Linsey Marr, associate professor of civil and environmental engineering at Virginia Tech.

The viruses survived best at low humidity, such as those found indoors in the winter, and at extremely high humidity. Humidity affects the composition of the fluid, namely the concentrations of salts and proteins in respiratory droplets, and this affects the survival rates of the flu virus.