COVID-19: Omicron displays reduced pathogenicity in hamster model

The Omicron variant displays reduced ability to cause disease compared to the Delta variant and an ancestral SARS-CoV-2 strain in a hamster model, according to data presented in a Nature paper published today.

Kei Sato and colleagues first investigated the ability of Omicron to fuse with human cells in a cell-based fusion assay, and found that Omicron displayed lower cell fusion compared to Delta and an ancestral SARS-CoV-2 variant (B.1.1). The authors suggest this may be due to poor spike-protein cleavage efficacy, which facilitates cell fusion, in Omicron.

The authors then investigated the pathogenicity of Omicron in a hamster model. Hamsters infected with Omicron exhibited significantly decreased weight loss compared to hamsters infected with Delta and ancestral SARS-CoV-2, although they did display significant weight loss compared to uninfected hamsters. Omicron-infected hamsters also showed less-impaired lung function compared to those infected with the other variants, and on some measures (such as subcutaneous oxygen saturation) compared favourably to uninfected hamsters. Omicron-infected hamsters also displayed significantly fewer lung lesions than hamsters infected with other variants.

The authors also investigated viral production by collecting daily oral swabs from infected hamsters. They observed that hamsters infected with Delta or ancestral SARS-CoV-2 saw the greatest viral replication one day after infection, with high levels of viral RNA maintained for a week. By contrast, Omicron-infected hamsters displayed peaks of viral replication at two to three days after infection, after which viral RNA loads rapidly declined.