Original title: Microscopic dynamics of contagion using active Brownian particles: universal scaling and propagation controlled by protection
Isela Sicarú Regalado-Alvarado, Francisco Alarcón · September 2026
Standard epidemic models often miss how individual movement and physical space affect disease spread. To fix this, researchers simulated contagion using self-propelled particles that move around and bump into each other. In this setup, a set portion of the population is protected from infection. They tested different crowd densities and fractions of protected agents. They discovered two distinct spread patterns separated by a crossover protection level of approximately 30%. Below 30% protection, the top spread rate depends on density, following a universal pattern driven simply by how often individuals bump into one another. Above 30%, this universal pattern breaks down. Here, protection takes over as the main control, suppressing the disease most effectively in low-density crowds.
Imagine a crowded dance floor where people bump into each other. Some dancers wear a special badge that prevents them from getting bumped. If fewer than 30% have badges, the number of bumps depends only on how many dancers are on the floor. Here, the dancers are individuals, the bumps are disease transmissions, and the badges are protection measures. The researchers found that once more than 30% have badges, the badges themselves control the bumping, especially when the floor is not too crowded.
This work shows that the effectiveness of protection measures like masks or distancing depends critically on population density and coverage level. Below 30% protection, disease transmission is driven by how often people meet, not by protection. Above that threshold, protection becomes the dominant factor, especially in sparse crowds. This helps policymakers design targeted interventions based on local density and coverage.
The abstract does not specify the exact mechanism of protection (e.g., immunity, masking) or the duration of protection. The model uses idealized active Brownian particles, which may not capture all real-world human movement patterns. The crossover at 30% is identified for this specific model; its universality across different diseases or social behaviors is not established. The study focuses on maximum contagion rate, not on long-term epidemic dynamics or eradication thresholds.
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Microscopic dynamics of contagion using active Brownian particles: universal scaling and propagation controlled by protection — Isela Sicarú Regalado-Alvarado, Francisco Alarcón
Abstract licence: CC0
Explanation generated by Synthesis from the abstract — it is not the original paper. This is a preprint, so it hasn't necessarily been peer reviewed.