If you’re one of the nearly 7 million people who suffered from influenza last season, you might be curious why the virus hits so much harder during winter months. A study recently published in Proceedings of the National Academy of Sciences by Yale researchers revealed a key reason why so many are afflicted and potentially die from flu during winter: low humidity.
Viral transmission from host to host can result from a combination of dry air and cold temperatures, which is something scientists have long understood. A research team from Yale—led by Akiko Iwasaki, Professor of Immunobiology—aimed to explore the effect of decreased humidity on the immune’s system ability to fight a flu infection. Their results, in short, showed that dry air compromises a host’s ability to stop an influenza virus infection. In a statement from Yale, Iwasaki stated: “If our findings in mice hold up in humans, our study provides a possible mechanism underlying this seasonal nature of flu disease.”
To explore the impacts of low humidity on immune systems and the influenza virus, the research team used mice that were genetically modified to resist viral infections as humans do. The mice were all housed at the same temperature; however, some experienced average humidity and others low humidity. Then, they were exposed to the influenza A virus.
The mice exposed to the low humidity conditions were more susceptible to influenza, but also demonstrated three critical immune response failures. First, the low humidity prevented cilia (the hair-like structures in airways) from removing viral particles and mucus. Second, it reduced the ability of airway cells to repair damage to the lungs caused by the virus. And third, once the virus was able to breach the mucosal barrier, the innate immune defense signaling system—interferons—completely failed.
While this study demonstrated that low humidity is a main factor in the seasonal increase of influenza cases and mortality, there are other factors to consider through more research. The research team is confident that during the next flu season, afflicted individuals can reduce symptoms and speed their recovery by using humidifiers to increase water vapor in the air.