In the temperate regions, between the subtropics and the polar circles, temperatures are not “extreme”, not burning hot nor freezing cold (temperate means moderate). In these parts of the world, seasonal influenza virus outbreaks happen during the winter months — peaking between November and March in the Northern Hemisphere (i.e. All of continental Europe, North America) and between May and September in the Southern Hemisphere (i.e. Australia, Zealandia, Brazil).

Scientists have hypothesized several reasons for this. Maybe because people are inside more in cold weather, one theory holds, so the virus spreads more easily. Or maybe it’s because people are not out in the sun making Vitamin D, and their immune systems are weak. Or perhaps people travel for holidays at certain times of years, helping spread the virus. So far, not one of these theories has proved a winner.

Recently, a group of researchers from Yale University, Veterans Affairs Connecticut Healthcare System, and Howard Hughes Medical Institute has pinpointed a key reason why people are more likely to get sick and even die from flu during winter months: low humidity.

The Yale research team, led by Akiko Iwasaki, the Waldemar Von Zedtwitz Professor of Immunobiology, explored the question using mice genetically modified to resist viral infection like 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 the influenza A virus. The study was published in the Proceedings of the National Academy of Sciences (PNAS).

They found that low humidity hindered the immune response of the animals in three distinct ways: 1) it prevented cilia, which are hair-like structures in airways cells, from removing viral particles and mucus, 2) it also reduced the ability of airway cells to repair damage caused by the virus in the lungs, 3) in low humidity, interferons or signaling proteins released by virus-infected cells to alert neighboring cells to the viral threat do not function optimally. The study offers 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 Akiko Iwasaki.

While the researchers emphasized 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 vapor 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. In addition, it’s always a good idea to wash your hands often and get vaccinated with the flu vaccine.