Comparative infection and pathogenesis of SARS-CoV-2 Omicron and Delta variants in aged and young Syrian hamsters

Coronavirus disease 2019 continues to batter the world with the unceasing introduction of new variants of the causative virus, SARS-CoV-2. In order to understand differences in disease caused by variants of concern and to develop variant-specific vaccines, suitable small animal models are required that mimic disease progression in humans at various stages of life. In this study, we compared the dynamics of infection with two SARS-CoV-2 variants of concern (Delta and Omicron) in aged (>1 year 3 months old) and young (<5 weeks old) Syrian hamsters ( Mesocricetus auratus ). We show that no weight loss occurred in Omicron infected groups regardless of age, while infection with the Delta variant caused weight loss of up to 10% by day 7 post-infection with slower and incomplete recovery in the aged group. Omicron replicated to similar levels as Delta in the lungs, trachea and nasal turbinates, with no significant differences in the tissue viral loads of aged versus young animals for either variant. In contrast to rare necrosis observed in Omicron-infected animals regardless of age, severe necrosis was observed in the olfactory epithelium in Delta-infected animals. Omicron infection also resulted in mild pulmonary disease in both young and aged animals compared to the moderate acute necrotizing bronchointerstitial pneumonia seen in Delta-infected animals. These results suggest that Omicron infection results in an attenuated clinical disease outlook in Syrian hamsters compared to infection with the Delta variant irrespective of age. ### Competing Interest Statement The authors have declared no competing interest.