Lower Humidity Linked to More Severe Flu Illness

Can using a humidifier help prevent you from getting the flu? Maybe so, according to new study on the link between low humidity and influenza virus severity.

Studies have shown there is a link between climate and influenza, and that decreases in humidity levels common in winter months can be a driver during flu season. In a study published online in the Proceedings of the National Academy of Sciences (PNAS), investigators cited previous findings that a drop in absolute humidity, which is dependent on relative humidity and temperature, correlates most closely with the rise in seasonal influenza-related deaths. In this study the investigators aimed to look further into the effects of low ambient humidity on enhancing influenza virus transmission.

“Before our study, while it was known that winter climate is associated with the transmission of flu, the impact of humidity on how the host responds to flu once we are infected by the virus was unknown,” said study co-author Eriko Kudo, who is a post-doctoral associate in the Department of Immunobiology, in an interview with Contagion®.

The authors note that the respiratory mucosal barrier provides the first line of defense against influenza and if the virus breaches this barrier it triggers innate immune defense mechanisms. Using Mx1 congenic mice genetically modified to have human resistance to viral infection, the study team examined the impacts of relative humidity on the mice’s response to influenza A virus infection and disease outcomes. The mice were kept in chambers at either a low relative humidity of 10-20% or at higher relative humidity of 50%.

The study team found that the mice kept in low humidity conditions had impaired defenses and were more susceptible overall to influenza illness and experienced more severe flu symptoms than mice kept in higher relative humidity. Lower levels of relative humidity impaired mucociliary clearance and tissue repair and blocked the induction of the interferon-stimulated genes known to restrict influenza A viruses, leading to higher viral burden. In addition, dry air exposure made mice more susceptible to disease mediated by inflammasome caspases.

“It’s well known that where humidity drops, a spike in flu incidence and mortality occurs,” said, Akiko Iwasaki, PhD, the Waldemar Von Zedtwitz Professor of Immunobiology at Yale University and study lead author in a recent statement. “If our findings in mice hold up in humans, our study provides a possible mechanism underlying this seasonal nature of flu disease.”

In their paper the investigators note that though host defenses against influenza are not impacted by humidity in all situations, influenza virus can thrive in the warm, wet conditions of tropical and subtropical settings.

“Future studies will be required to understand why certain regions of the world may be affected differently by humidity and temperature,” write the authors, adding that the use of humidifiers in the winter months and flu season may have benefits. “Our study suggests that increasing ambient humidity may be a viable strategy to reduce disease symptoms and to promote more rapid recovery in influenza-infected individuals.”