Science: Urbanization & Humidity Shape Intensity of Influenza Epidemics In U.S. Cities

#13,566


While we study how influenza viruses evolve and change over time in order to assess their threats to public health, at the same time we - as a society - are changing as well.

In the 100 years since the last great pandemic - which claimed between  50 million to 100 million lives - our global population has surged from under 2 billion to more than 7.5 billion, and we've seen an unprecedented movement towards urbanization - including into high density mega-cities - around the globe.

More people work indoors today, in a more or less `controlled' environment, and in close proximity to others.  Many commute - particularly in urban settings - in crowded public conveyances.  Schools, hospitals, workplaces, sporting events . . . practically any place people gather today . . . are larger, and more crowded.

Add in other environmental changes around the world, along with a highly mobile society aided and abetted by millions of international flights each year and a modern highway system, and the conditions afforded the influenza viruses of today are far different from what faced the 1918 virus.

And as last year's deadly H3N2-centric flu season showed, it doesn't take a pandemic in order to kill tens of thousands of Americans (see CDC: More Than 900,000 Hospitalizations & 80,000 Deaths In Last Winter's Flu Season).

Understanding what factors - including changes in how and where we live - affect influenza transmission, is vital if we hope to mitigate the next pandemic in a meaningful way. 

Particularly since one pandemic plan won't likely fit all locations. What works in NYC or Chicago, might fail in Abilene or Orlando.

All of which brings us to a new study, published yesterday in Science, which examines the arrival, and duration of seasonal influenza over 6 years (2002-2008) across 603 U.S. cities, and find major differences depending upon both size and population density, and humidity.

Simply put, flu epidemics arrive earlier, and last longer, in large urban centers. Conversely, smaller cities tend to have shorter, but sharper (more intense), flu epidemics.

While your overall risk of contracting the flu in a larger city doesn't appear to be any greater, the duration of a pandemic (or seasonal) influenza wave increases. 

While a `flatter curve' is desirable in terms of an epidemic's impact on hospitals and other essential services (see Community Pandemic Mitigation's Primary Goal : Flattening The Curve), larger cities may provide a flu virus more time to smolder and greater opportunities to evolve or spread.

The study also found that larger, densely populated cities, are less dependent than smaller cities upon lower humidities (see bioRxIv: Humidity As A Non-pharmaceutical Intervention For Influenza A) to promote the spread of influenza.  

First a link to the report, then I'll return with some links to summaries and a postscript.  Follow the link to read the open-access report in its entirety.

Report
Urbanization and humidity shape the intensity of influenza epidemics in U.S. cities
Benjamin D. Dalziel1,2,*, Stephen Kissler3, Julia R. Gog3, Cecile Viboud4, Ottar N. Bjørnstad5, C. Jessica E. Metcalf6,7,  Bryan T. Grenfell4,6,7

Science 05 Oct 2018:
Vol. 362, Issue 6410, pp. 75-79
DOI: 10.1126/science.aat6030

Seasonal flu by ZIP code

Influenza virus strikes communities in northern latitudes during winter, straining health care provision almost to the breaking point. Change in environmental humidity is a key driver, but many other seasonal and social factors contribute. Dalziel et al. obtained a geographical distribution of doctor visits for influenza-like illness for more than 600 U.S. cities (see the Perspective by Wallinga).

Some ZIP codes regularly experienced sharply defined peaks of cases, or intense epidemics, and others showed a longer, more diffuse influenza season. The surges tended to occur in smaller cities with less residential density and lower household incomes. Larger, more densely populated cities had more-diffuse epidemics, presumably because of higher rates of personal contact, which makes influenza transmission less subject to climate variation.

Science, this issue p. 75; see also p. 29

Abstract

Influenza epidemics vary in intensity from year to year, driven by climatic conditions and by viral antigenic evolution. However, important spatial variation remains unexplained. Here we show predictable differences in influenza incidence among cities, driven by population size and structure. Weekly incidence data from 603 cities in the United States reveal that epidemics in smaller cities are focused on shorter periods of the influenza season, whereas in larger cities, incidence is more diffuse. Base transmission potential estimated from city-level incidence data is positively correlated with population size and with spatiotemporal organization in population density, indicating a milder response to climate forcing in metropolises. This suggests that urban centers incubate critical chains of transmission outside of peak climatic conditions, altering the spatiotemporal geometry of herd immunity.

(Continue . . . )

You'll find some informative summaries at:

City size and structure may influence influenza epidemics

New research could lead to more accurate predictions about flu seasons

By Aimee Cunningham

3:13pm, October 4, 2018

 and

Population, Transportation and Climate Impact A City’s Flu Season
5 October 2018
Meagan Phelan

As of 2017, there are 47 Megacities around the world (pop > 10 million), with the bulk of them in Asia. The combined population of the largest ten would make it the 3rd largest country in the world.

According to the United Nations World Urbanization Prospects: The 2018 Revision:

• Globally, more people live in urban areas than in rural areas, with 55 % of the world’s population residing in urban areas in 2018. In 1950, 30 % of the world’s population was urban, and by 2050, 68 % of the world’s population is projected to be urban.
• Today, the most urbanized regions include Northern America (with 82 % of its population living in urban areas in 2018), Latin America and the Caribbean (81 %), Europe (74 %) and Oceania (68%). The level of urbanization in Asia is now approximating 50 %. In contrast, Africa remains mostly rural, with 43 % of its population living in urban areas.
• The rural population of the world has grown slowly since 1950 and is expected to reach its peak in a few years. The global rural population is now close to 3.4 billion and is expected to rise slightly and then decline to around 3.1 billion in 2050. Africa and Asia are home to nearly 90 % of the world’s rural population. India has the largest rural population (893 million), followed by China (578 million).
• The urban population of the world has grown rapidly since 1950, having increased from 751 million to 4.2 billion in 2018. Asia, despite being less urbanized than most other regions today, is home to 54 % of the world’s urban population, followed by Europe and Africa (13 % each).
• Growth in the urban population is driven by overall population increase and by the upward shift in the percentage living in urban areas. Together, these two factors are projected to add 2.5 billion to the world’s urban population by 2050, with almost 90 % of this growth happening in Asia and Africa.

While we have antiviral drugs, antibiotics, a wealth of medical knowledge, and infrastructures that far eclipse what was available in 1918 - not all of the advantages of our modern society go to our favorite host species.

Crowded cities and public conveyances, artificial environments, CAFO based food production, our encroachment into previously wild areas of the planet, increased international travel, and other anthropogenic changes have radically changed the `panscape' for pathogens over the last century. 

All of which makes our ability to well mitigate the next great pandemic far from assured.