Drier Winter Air May Propel Flu's Spread

WEDNESDAY, May 15, 2019 (HealthDay News) -- Ever wonder why the flu rarely strikes in the summer?

New research offers an explanation: That dry air your heating system sends throughout your home in the winter weakens your immune system, making you a prime target for viral infection.

Cold temperatures and low humidity were known to increase transmission of flu viruses, but exactly how low humidity affects the immune system's defenses against the flu wasn't well understood.

In this study, Yale University researchers used mice that were genetically modified to resemble the way humans resist viral infections.

The mice were kept in chambers of equal temperature, but with either low or normal humidity, and exposed to the influenza A virus.

Low humidity hindered the immune response of the mice in three ways, according to the study published May 13 in the journal Proceedings of the National Academy of Sciences.

It prevented hair-like structures (cilia) in airways from removing viral particles and mucus. It also lowered the ability of airway cells to repair damage caused by the flu virus in the lungs.

Low humidity also compromised an immune defense system involving interferons, proteins released by virus-infected cells to alert neighboring cells to the viral threat.

The findings help explain why more people get the flu when the air is dry, the study authors said.

"It's well known that where humidity drops, a spike in flu incidence and [flu-related death] occurs. If our findings in mice hold up in humans, our study provides a possible mechanism underlying this seasonal nature of flu disease," team leader Akiko Iwasaki said in a news release from Yale, where she is a professor of immunobiology.

While humidity isn't the only factor in the spread of the flu, it's an important one that needs to be considered during the winter, according to the researchers.

Increasing moisture levels in the air with humidifiers at home, and in school, work and hospital environments could reduce flu symptoms and speed recovery, they suggested.

More information

The U.S. Centers for Disease Control and Prevention has more on the flu.