A single-residue deletion in H9N2 hemagglutinin uncouples viral burden from disease severity.

A single-amino-acid deletion at HA residue 227 in H9N2 influenza isogenic viruses reduces replicative fitness yet is linked to more severe pulmonary lesions despite lower respiratory shedding.

• Why it matters: Viral burden is often used to predict disease severity, but rare HA deletions may uncouple replication from pathology through changes in HA–NA function and host immunity.
• What they did: Using isogenic reverse-genetics H9N2 viruses carrying HAΔ227, the study measured HA receptor-binding avidity, apparent virion-associated NA activity, HA/NA functional balance, in vitro replication, and in vivo respiratory shedding in chickens, alongside cytokines and lesion severity.
• The result: HAΔ227 increased IFN-γ, IL-10, IL-1β, and MCP-1 in infected chickens and these immune changes were associated with more severe pulmonary lesions, indicating disease severity may not track viral RNA load.

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