Shaping seasonal flu epidemics: City density, and climate

The more people a city has and the more organized its inhabitants' movements, the longer its flu season will last, even if climatic conditions aren't otherwise favorable for flu to spread, a new report suggests. The report's findings - which "imply a shift in thinking about infection control" according to a related Perspective - represent an important step towards better predicting outbreak trends for influenza, which sickens millions in the U.S. annually, and kills tens of thousands. Most recently, the 2017-2018 winter flu season in the U.S., as one example, was particularly long and severe, challenging the country's healthcare system. Flu spreads and evolves in a range of animals; forecasting and controlling this is important for public health. However, various factors - including those related to immunity, social crowding, shifting climatic conditions, and more - have been proposed to explain the size and shape of seasonal influenza epidemics, and inferring the relative importance of each one is challenging. To more accurately predict flu outbreaks, researchers need to look at an array of processes, from urbanization to climate.

Here, working towards more accurate seasonal flu predictions, Benjamin Dalziel and colleagues analyzed weekly flu incidence data (as represented by data on U.S. patients visiting doctors' offices with influenza-like illness between 2002 and 2008) from more than 600 U.S. regions of varying size and with varying structures, including transportation patterns. Following their analyses, they report that influenza spreads differently in urban centers with pockets of high population density connected by organized movement. In such metropolises, flu cases were more diffuse through the winter months - including early and late in the season when the weather is not optimal for flu transmission. By contrast, in smaller cities, flu cases were more often tightly grouped in a short period during peak season. The different patterns of seasonal flu epidemic the authors observed by geographic region - like the tendency for flu cases to be more tightly grouped in the southeast U.S. - persisted year after year, they say. Hypothesizing that such patterns might be caused in part by responses to climate factors, the researchers evaluated the influence of a key weather metric - specific humidity; as it decreases, the moisture droplets that flu-infected people cough out remain viable in the air (outside their host) for longer. Modeling revealed how local humidity patterns in key regions contributed to differences in flu patterns there, with flu epidemics being more intense in cities with larger humidity shifts. Perhaps counterintuitively, it was the cities with smaller population sizes where humidity shifts played a bigger role; in those locations, flu transmission merely because of high population density is slightly lessened, and as such, decreased humidity is more of a boon to flu virus spread. The work is "important for policy makers," Jacco Wallinga writes in a related Perspective, "because it would indicate that metropolitan areas should focus on reducing influenza spread, whereas small towns should focus on reducing harm."