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Moderate-Sized Cities and the 2009 H1N1 Pandemic

Amesh A. Adalja, MD, FACP, FACEP, FIDSA, January 10, 2019

It is widely believed that major cities—especially megacities—play an outsized role in the transmission of infectious diseases. Historical examples abound of cities being inundated with infectious disease outbreaks, with residents fleeing the cities. Much of our public health emergency infrastructure is concentrated in major cities. However, new research suggests that in the last flu pandemic, which occurred in 2009, moderate-sized cities served as transmission hubs for the fall wave of the virus.

Medical Claims Database Used

In a paper published in Epidemics, Kissler and colleagues employed the IMS medical claims database, which covered about 50% of all outpatient visits in 2009, to determine where cases of influenza occurred during the fall. Zip codes (limited to 3 digits) were used for city identification, and ICD codes consistent with influenza, fever and respiratory symptom, or febrile viral illness were used to identify cases of influenza. These data were used to plot various epidemic onset times throughout the country. A mathematical model that optimally fit the data was constructed, and a forward transmission network was constructed.

Hubs in Moderate-Sized Cities

Using the mathematical model, 4 transmission hubs were identified. The 2 largest hubs were in Grenada, Mississippi, and Albany, Georgia. The populations of those 2 regions were 113,782 and 111,263, respectively. Interestingly, these 2 areas have among the earliest school start dates, which may provide a partial explanation. Stockton, California, and Omaha, Nebraska, were also identified as major hubs of transmission. Fully 75% of fall cases in the database could be explained via seeding from 1 of the identified hubs.

Reexamining the Role of Mid-Sized Cities in Epidemic Transmission

If Kissler and colleagues’ mathematical modeling truly reflects the reality of the dynamics of the 2009 influenza pandemic, it has important implications. While it is indisputable that for certain infectious diseases, the role of megacities and international connectivity cannot be ignored, the 2009 influenza pandemic appears to have relied less on them in its march across the nation. There are caveats to the data used, which may have influenced results, such as the incompleteness of the database, the lack of specificity of some ICD codes used, and the exclusive reliance on outpatient diagnoses.

Given the caveats and the limitations of mathematical models, if moderate-sized cities played an outsized role in transmission, it reinforces the need to fortify public health defenses against influenza in many areas that are not traditionally thought of as areas of major public health concern. When the next pandemic of influenza emerges, it will be difficult to predict in real time which geographic locations will drive spread, but knowing the transmission dynamics in 2009 may help challenge assumptions and influence resource allocation. Narrative accounts of the fall of 2009 from these locales would also be of value in understanding the idiosyncrasies of their experience with the 2009 pandemic.


Kissler SM, Gog JR, Viboud C, et al. Geographic transmission hubs of the 2009 influenza pandemic in the United States. Epidemics 2018. Accessed January 9, 2019.