Researchers have long pondered why influenza outbreaks most often occur in the winter. Some think it is the onset of the school year, others the amount of light produced by the sun, and some think it is relative humidity, but none of these explanations provides a very good basis for modeling flu outbreaks. A new study reported Monday in the journal PLoS Biology indicates that the best correlation is with absolute humidity -- a measure of the actual amount of water vapor in the air, compared with relative humidity, which is a measure of how much is in the air compared to how much the air can hold. Absolute humidity is lowest in winter in most temperate regions of the world and much higher in the summer.
A variety of laboratory studies have shown that flu viruses become less infective when there are higher quantities of water in the air. To validate these findings in humans, a team headed by atmospheric scientist Jeffrey Shaman of Oregon State University used 31 years of absolute humidity data for several states, then for the country as a whole, in mathematical models of influenza spread. In both cases, they found a much closer correlation with absolute humidity than with any other variable they studied. A dry period was not an absolute requirement for triggering a seasonal outbreak, but it was present in 55% to 60% of the outbreaks. "The virus response is almost immediate," Shaman said in a statement. "Transmission and survival rates increase about 10 days later. Observed mortality rates follow."
The sesonality of flu may affect that of other infectious diseases, the researchers said, because the flu virus lowers immune response. Researchers don't know, however, how higher humidity reduces virus transmission.
-- Thomas H. Maugh II