A modeling study integrating high-resolution influenza case data with human behavioral patterns across US counties reveals that flu seasonality emerges primarily from behavioral adaptations to weather—particularly weather-driven indoor crowding—rather than environmental factors like cold air, humidity, or viral aerosol survival alone. Southeastern counties sustain persistent summer influenza circulation due to heat-driven indoor congregation and built-environment vulnerabilities, seeding nationwide epidemics as early as August. A transmission model incorporating intercounty connectivity, school calendars, and building-modulated transmissibility successfully reproduces this spatiotemporal invasion pattern.
This finding reframes decades of flu seasonality research, which has largely centered on aerosol physics and viral survival under cold, dry conditions. The behavioral crowding hypothesis has existed at the margins of the field, but this analysis provides a spatially granular, mechanistically grounded framework that elevates it to a primary driver. For public health practice, the implications are actionable: if southern reservoirs seed northern epidemics, vaccination campaigns timed to precede local southeastern onset—potentially in late summer—could meaningfully reduce national burden. This challenges the conventional autumn vaccination push.
Critical limitations deserve attention: this is a preprint not yet peer-reviewed, so conclusions should be treated as preliminary. The reliance on modeled transmissibility and behavioral proxies introduces assumptions that peer review may scrutinize. Whether the framework generalizes beyond the US or to other respiratory pathogens remains untested. Still, the paradigm shift from environment-centric to behavior-centric seasonality explanations, if validated, carries significant preparedness implications.