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Expand chart
Data: Dalziel, et. al., 2018, "Urbanization and humidity shape the intensity of influenza epidemics in U.S. cities"; Chart: Andrew Witherspoon/Axios

Smaller cities — especially those that aren't very humid — tend to have higher flu epidemic intensity than larger cities, scientists say in a new study published in Science on Thursday. Conversely, large cities with focused transportation systems tend to have longer, more consistent flu seasons, they found.

Why it matters: As seen last season, flu epidemics can be quite deadly and their intensity is caused by several factors. When smaller cities, like Nashville and Atlanta shown above, experience sharp spikes in flu cases, there is a bigger burden placed on their health care systems, which may call for implementing "surge capacity" in treatment centers, study author Cecile Viboud said during a press conference call.

Background: Seasonal flu epidemics are complicated and vary based on a number of factors — not the specifics of the population and the virus (like exact virus types and vaccination coverage) but there's also growing evidence that "outside factors" may play a role.

  • Some of these include population density, commuting patterns and climate (in particular, "specific humidity," low levels of which have been shown to allow virus droplets to remain viable in the air for longer).
  • This new study examines the role such outside factors play.

What they did: The researchers looked at 6 years (2002–2008) of medical claims for weekly incidence of influenza-like illnesses in 603 regions of the U.S. They compared the levels of such illnesses with factors believed to play a role in transmission: population size, humidity, residential crowding, median household, vaccination coverage and income.

What they found:

  • Epidemic intensity (or the peak level of infections) is actually higher in cities with smaller populations, less residential crowding and lower incomes, co-author Benjamin Dalziel tells Axios.
  • Densely populated, upper-income, larger cities, on the other hand, showed lower epidemic intensity despite being a "transmission hub." While infection rates were higher overall, they were spread out over a longer period of time, enabling the health care system to respond better, Dalziel says.
  • In large cities, what impacts epidemic intensity is the organization of movement patterns and the "hot pockets" of infection.
  • In small cities, humidity plays a bigger role, Dalziel says, because the virus droplets have more time to infect people.

Outside perspective: Other researchers say this is an interesting look at possible factors affecting flu intensity, and should inspire further research. Katelyn Gostic, disease ecologist and Ph.D candidate at UCLA, tells Axios the study shows interesting patterns.

"[T]his study shows that influenza epidemics in smaller cities burn hot and fast, while epidemics in big megacities burn at more of a year-round smolder," she said.

"In smaller cities, it looks like population density is low enough that flu viruses need optimal climatic conditions to really ignite a chain of transmission. But in bigger cities, it looks like population density is high enough that the flu virus can spread even under non-ideal climatic conditions, although a drop in humidity still does boost transmission a little bit."
— Katelyn Gostic, UCLA

Big picture: Viboud stresses that the study "does not show that some cities are safer than others for flu" — just that different types of patterns are emerging, which tended to be consistent for that city over that time period. And Jeffrey Shaman, director of Columbia University's climate and health program, said the results don't yet provide enough information to be useful for providing flu control solutions.

What's next: Jacco Wallinga, of the National Institute for Public Health and the Environment in the Netherlands, writes in Science Perspectives:

"This is important for policy-makers because it indicates that metropolitan areas should focus on reducing influenza spread, whereas small towns should focus on reducing harm."

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