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BETHESDA, Md. — Global warming could spell the end of flu seasons, research released Friday by the National Institution of Health suggests, because the influenza virus better propagates and is more sustainable in colder climates.
Two types of environmental conditions — cold-dry and humid-rainy — are associated with seasonal influenza epidemics, according to an epidemiological study released Friday that was led by researchers at the NIH's Fogarty International Center. The paper, published in PLoS Pathogens, presents a simple climate-based model that maps influenza activity globally and accounts for the diverse range of seasonal patterns observed across temperate, subtropical and tropical regions.
After assessing the role of local climatic variables on virus seasonality in a global sample of study sites, Viboud and her colleagues found that temperature and specific humidity were the best individual predictors of the months of maximum influenza activity, known as influenza peaks. The team discovered that in temperate regions, influenza was more common one month after periods of minimum specific humidity. These periods happen to coincide with months of lowest temperature. In contrast, sites that maintained high levels of specific humidity and temperature were generally characterized by influenza epidemics during the most humid and rainy months of the year. “The models we used predicted the timing of peak influenza activity with 75% to 87% accuracy,” stated Cecile Viboud, who headed the study.
Data from animal studies indicate low temperature and humidity increase the duration of the virus’s reproduction and expulsion in infected organisms and virus stability in the environment, increasing the probability of transmission through coughing, sneezing or breathing. In contrast, high temperature seems to block airborne transmission.
Human influenza infections exhibit a strong seasonal cycle in temperate regions, and laboratory experiments suggest that low specific humidity facilitates the airborne survival and transmission of the virus in temperate regions. Specific humidity is the ratio of water vapor to dry air in a particular body of air while relative humidity — commonly used in weather forecasts — is the amount of water vapor in the air relative to its capacity to hold water vapor, and is primarily a function of temperature.
According to James Tamerius, a geographer at Columbia University and the first author of the study, the effect of low specific humidity on influenza could cause annual winter epidemics in temperate areas. “However, this relationship is unlikely to account for the epidemiology of influenza in tropical and subtropical regions where epidemics often occur during the rainy season or transmit year-round without a well-defined season,” he said.
The findings could be used to improve existing current influenza transmission models, and could help target surveillance efforts and optimize the timing of seasonal vaccine delivery, according Viboud. “The model could have a broader application, encouraging researchers to analyze the association between climatic patterns and infectious disease across a wide range of diseases and latitudes,” Viboud said.
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