Climates Associated with Seasonal Flu Epidemics

Two types of environmental conditions — cold-dry and humid-rainy — are associated with seasonal influenza epidemics, according to an epidemiological study led by researchers at the National Institutes of Health’s Fogarty International Center. The study, 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.

The researchers used a recently developed global database that provides information on influenza peaks from 1975 to 2008 for 78 sites worldwide, spanning a range of latitude between 1 and 60 degrees, with 39 percent of the sites located in the tropics. To ensure independent validation, they also used epidemiological data from nine countries participating in FluNet, the World Health Organization’s global influenza surveillance program. The nine countries included Spain, Tunisia, Senegal, Philippines, Vietnam, Colombia, Paraguay, South Africa and Argentina, none of which was represented in the original 78-location database, and were chosen because each country provided several years of data.

They found that temperature and specific humidity were the best individual predictors of the months of maximum influenza activity, known as influenza peaks. Specifically, they discovered that in temperate regions, influenza was more common one month after periods of minimum specific humidity, which 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. “Anecdotal evidence suggests that colder climates have winter flu, while warmer climates that experience major fluctuations in precipitation have flu epidemics during the rainy season, and the current study fits that pattern,” said Cecile Viboud, PhD, who headed the study. “In contrast, the seasonality of influenza is less well-defined in locations with little variation in temperature and precipitation, and is a pattern that remains poorly understood. One hypothesis that is often used to explain tropical influenza activity is that people congregate indoors more frequently during the rainy season, increasing contact rates and disease transmission.”

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.

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 to Viboud.