Pulmonary lesions following inoculation with the SARS-CoV-2 Omicron BA.1 (B.1.1.529) variant in Syrian golden hamsters

The Omicron BA.1 (B.1.1.529) SARS-CoV-2 variant is characterized by a high number of mutations in the viral genome, associated with immune escape and increased viral spread. It remains unclear whether milder COVID-19 disease progression observed after infection with Omicron BA.1 in humans is due to reduced pathogenicity of the virus or due to pre-existing immunity from vaccination or previous infection. Here, we inoculated hamsters with Omicron BA.1 to evaluate pathogenicity and kinetics of viral shedding, compared to Delta (B.1.617.2) and to animals re-challenged with Omicron BA.1 after previous SARS-CoV-2 614G infection. Omicron BA.1 infected animals showed reduced clinical signs, pathological changes, and viral shedding, compared to Delta-infected animals, but still showed gross- and histopathological evidence of pneumonia. Pre-existing immunity reduced viral shedding and protected against pneumonia. Our data indicate that the observed decrease of disease severity is in part due to intrinsic properties of the Omicron BA.1 variant.

奥密克戎BA.1（B.1.1.529）严重急性呼吸综合征冠状病毒2型（SARS-CoV-2）变异株以病毒基因组存在大量突变为特征，这类突变与免疫逃逸及病毒传播能力增强相关。目前尚不明确人类感染奥密克戎BA.1后观察到的新冠病情减轻现象，究竟源于该病毒致病性降低，还是源于接种疫苗或既往感染所带来的预存免疫。本研究通过对仓鼠接种奥密克戎BA.1，以评估其致病性与病毒排毒动力学，并与德尔塔（B.1.617.2）变异株感染组、以及既往感染SARS-CoV-2 614G后再次用奥密克戎BA.1攻毒的动物组进行对比。与德尔塔感染组相比，奥密克戎BA.1感染的仓鼠表现出更轻微的临床症状、病理改变与病毒排毒水平，但仍存在肺炎的大体及组织病理学证据。预存免疫可降低病毒排毒量，并对肺炎起到保护作用。本研究数据表明，此前观测到的病情严重程度降低现象，部分源于奥密克戎BA.1变异株自身的内在特性。