Public health scientists at the Columbia University Mailman School of Public Health used advanced computer simulations to retrace how the 2009 H1N1 flu pandemic and the 2020 COVID-19 pandemic spread across the United States. Their analysis shows just how quickly respiratory pandemics can move and why stopping outbreaks early is so difficult. Published today (January 5) in the journal Proceedings of the National Academy of Sciences, the research is the first study to directly compare how these two major U.S. pandemics spread across metropolitan areas nationwide.

A Look Back at Two Major U.S. Pandemics

The impact of both outbreaks was severe. In the United States, the 2009 H1N1 flu pandemic led to 274,304 hospitalizations and 12,469 deaths. The COVID-19 pandemic has had an even greater toll, with 1.2 million confirmed deaths so far.

The research team set out to better understand how these pandemics traveled from place to place in order to improve preparedness for future outbreaks. To do this, they combined detailed information about how each disease spreads with computer models that incorporated air travel, daily commuting patterns, and the possibility of superspreading events. Their simulations covered more than three hundred metropolitan areas across the U.S.

Rapid Spread Before Detection

The results showed that both pandemics were already circulating widely in most major metro areas within just a few weeks. This rapid expansion often happened before early case detection or government response measures were in place. Although the exact routes of transmission differed between H1N1 and COVID-19, both relied on key urban hubs to fuel nationwide spread. Cities such as New York and Atlanta played major roles. Air travel emerged as the dominant factor driving spread, far more than commuting, but unpredictable transmission patterns made it extremely difficult to forecast where outbreaks would surge next.

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