Researchers have found that low humidity is a key reason why people are more likely to get sick and even die from flu during winter.
While experts know that cold temperatures and low humidity promote transmission of the flu virus, less is understood about the effect of decreased humidity on the immune system's defences against flu infection.
The Yale research team, led by Akiko Iwasaki, explored the question using mice genetically modified to resist viral infection as humans do. The study was published in the journal Proceedings of the National Academy of Sciences (PNAS).
The study offered insight into why the flu 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 Iwasaki.
The mice were all housed in chambers at the same temperature, but with either low or normal humidity. Then they were exposed to the influenza A virus.
The researchers found that low humidity hindered the immune response of the animals in three ways. 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 signalling proteins released by virus-infected cells to alert neighbouring cells to the viral threat. In the low-humidity environment, this innate immune defence system failed.
While the researchers emphasised that humidity is not the only factor in flu outbreaks, it is an important one that should be considered during the winter season. Increasing water vapour in the air with humidifiers at home, school, work, and even hospital environments is a potential strategy to reduce flu symptoms and speed recovery, they said.