Everyone knows that winter is flu season, but until now scientists didn’t know why influenza cases spiked during the colder months. They came up with plenty of hypotheses–for example, they proposed that people are more exposed when huddled indoors, and that lower melatonin and vitamin-D levels can weaken immune systems [ScienceNOW Daily News]–but none of the previous suggestions proved correct. Now, however, researchers say they have the answer: The influenza virus thrives in the cold, dry air of winter. “The correlations were surprisingly strong. When absolute humidity is low, influenza virus survival is prolonged and transmission rates go up” [AP], said lead researcher Jeffrey Shaman.
Researchers had previously looked for a link between flu rates and relative humidity, but Shaman says that absolute humidity (which measures the amount of water in the air, regardless of temperature) is the real key. Relative humidity varies with air temperature, because more moisture can be present in warm air than in cold…. What that means is warm air at 30 percent relative humidity and cold air at 60 percent relative humidity may actually have the same amount of water in the air. So, while the cold air sounds moist, it might be pretty dry — just what the flu likes [AP].
Since the virus can survive longer in cold, dry conditions, the chances are greater that someone will come along and be infected with the virus, experts say. Now that they know what kind of environment the virus likes, epidemiologist Marc Lipsitch says the next step is to study flu rates closer to the equator. “One really key question is how much influenza is transmitted in tropical locations”—places with high absolute humidity year-round—”and how this compares to temperate parts of the world,” he said [National Geographic].
To reach their conclusions, published in the Proceedings of the National Academy of Sciences [subscription required], researchers reanalyzed data from several old studies of whether relative humidity affected virus survival and transmission, and found that absolute humidity had a much stronger correlation with virus success. The new findings bolster the idea, suggested previously by others, that increasing humidity in nursing homes and emergency rooms might help prevent flu among vulnerable patients [ScienceNOW Daily News]. But experts say that health care facilities would have to strike a delicate balance, as more humid air might boost mold and other pathogens.