Bridging spatial scales in respiratory viral infection.

Respiratory viruses spread within the host through a combination of local expansion and occasional long-range dissemination that seeds new infection foci. We present LEAP (Lotka-Euler Airway Pathogen model), an analytically tractable within-host model that captures this two-scale process. The model takes the form of an age-dependent branching process and yields a closed-form expression for the exponential growth rate during early infection. Using empirical data to parameterize the model, we find that productive dissemination requires only a small number of successful long-range seeding events per infected cell, with distinct values for SARS-CoV-2 and influenza A virus. LEAP further predicts that, in these well-adapted viruses, interferon-mediated restriction only weakly affects exponential growth, while remaining decisive for poorly adapted pathogens. More broadly, the model provides a flexible framework for experimentally testable predictions of early infection dynamics.