Influenza and Low Humidity

Researchers have pinpointed a key reason why people are more likely to get sick and even die from Influenza during winter months: low humidity.

While experts know that cold temperatures and low humidity promote transmission of the Influenza Virus, less is understood about the effect of decreased humidity on the immune system’s defenses against an Influenza infection.

The research team explored the question using mice genetically modified to resist viral infection as humans do. The mice were all housed in chambers at the same temperature, but with either low or normal humidity. They were then exposed to an Influenza A Virus.

The researchers found that low humidity hindered the immune response of the animals in three ways[1]. It prevented cilia, which are hair-like structures in airways cells, from removing viral particles and mucus. It also reduced the ability of airway cells to repair damage caused by the virus in the lungs. The third mechanism involved interferons, or signaling proteins released by virus-infected cells to alert neighboring cells to the viral threat. In the low-humidity environment, this innate immune defense system failed.

The study offers insight into why Influenza is more prevalent when the air is dry. "It’s well known that where humidity drops, a spike in flu incidence and mortality occurs. If our findings in mice hold up in humans, our study provides a possible mechanism underlying this seasonal nature of flu disease," said lead scientist Iwasaki.

While the researchers emphasized that humidity is not the only factor in Influenza outbreaks, it is an important one that should be considered during the winter season. Increasing water vapor in the air with humidifiers at home, school, work, and even hospital environments is a potential strategy to reduce Influenza symptoms and speed recovery, they said.

[1] Kudo et al: Low ambient humidity impairs barrier function and innate resistance against influenza infection in PNAS – 2019. See here.