Hvid, U.; Nielsen, B. F.; sneppen, k.

Respiratory viruses spread within the host through both local expansion and occasional long-range dissemination that seeds new infection foci.We present LEAP, an analytically tractable within-host model that captures this two-scale process by coupling local plaque growth to long-range seeding. The model reduces to an age-dependent branching process and yields a closed-form expression for the exponential growth rate during early infection. Parameterization using plaque growth and viral load data suggests 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 interferon-mediated restriction may weakly affect exponential growth in these well-adapted viruses while remaining decisive for poorly adapted ones. More broadly, the model provides a flexible framework for experimentally testable predictions of early infection dynamics.