UC San Diego Study Finds Pandemic Viruses Rarely Adapt Before Human Spillover

Overview

• Published in Cell, the UC San Diego–led analysis surveyed genomes from influenza A, Ebola, Marburg, mpox, SARS‑CoV and SARS‑CoV‑2 focusing on evolution just before human outbreaks.
• Across outbreaks, selection pressures before spillover matched routine circulation in animal reservoirs, with detectable shifts typically arising only after sustained human transmission began.
• The team validated its approach using viruses adapted in cell culture or laboratory animals, which showed clear genomic signatures distinct from natural transmission.
• A notable exception emerged in the 1977 H1N1 reappearance, whose limited divergence and selection pattern align with laboratory propagation, consistent with long‑held suspicions about its origin.
• Authors propose the framework for outbreak forensics, viral surveillance and preparedness, while cautioning that absence of a lab‑selection signal cannot exclude every possible laboratory accident.