How COVID and H1N1 swept through U.S. cities in just weeks

New simulations reveal that both H1N1 and COVID-19 spread across U.S. cities in a matter of weeks, often before officials realized what was happening. Major travel hubs helped drive rapid nationwide transmission, with air travel playing a bigger role than daily commuting. Unpredictable transmission patterns made real-time forecasting especially difficult. The study highlights why early detection systems are critical for slowing future pandemics.