SEVA: An externally driven framework for reproducing COVID-19 mortality waves without transmission feedback

Understanding the dynamical mechanisms underlying epidemic wave formation remains a central problem in mathematical epidemiology. Population-level epidemic waves are commonly interpreted as emergent consequences of nonlinear transmission feedback between susceptible and infectious individuals. However, epidemic time series from different regions often display markedly different waveform regimes, ranging from sharply peaked epidemics with rapid post-peak decline to more prolonged plateau-like dynamics. Here we propose the SEVA (Seasonal/Environmental Viral Activity) framework as a parsimonious alternative dynamical interpretation of epidemic wave formation. In this formulation, epidemic waveforms arise from depletion of a finite vulnerable population under a temporally structured viral activity field. The activity function is represented by a monotonic logistic hazard describing the temporal evolution of viral activity. With activation timing and steepness held constant across regions, daily incidence emerges as the product of activity intensity and the remaining vulnerable population. The framework is applied to first-wave COVID-19 hospitalization and mortality data from selected European countries and U.S. states during spring 2020. With fixed activation parameters and region-specific activity intensity, the model provides a simple dynamical explanation for diverse epidemic waveform regimes--including sharply peaked waves and plateau-like dynamics--without modification of the underlying dynamical structure. When epidemic trajectories are expressed in normalized form, curves from regions with very different mortality burdens display closely similar temporal structures. Within the SEVA formulation, this behaviour arises naturally from the interaction between a common temporal activation profile and regionally varying activity intensity. In this perspective, sharply peaked epidemics and plateau-like trajectories represent different dynamical regimes of the same activity-driven depletion process.