Virology: Omicron exhibits reduced pathogenicity in mouse and hamster models

The Omicron variant has a reduced ability to cause infection and disease in preclinical rodent models, according to a paper, published in Nature.

As part of the collaborative network of the SARS-CoV-2 Assessment of Viral Evolution programme, Michael Diamond, Yoshi Kawaoka, Adrianus Boon and colleagues in multiple independent laboratories evaluated the ability of Omicron to cause infection and disease in mice and hamsters. In separate experiments, the authors inoculated different strains of mice with isolates of Omicron and observed that — unlike mice infected with previous variants — these mice did not lose weight. When comparing levels of the virus in the respiratory tract of mice, researchers found that levels of viral RNA and infectious virus were reduced for Omicron compared to Beta. For example, in one series of experiments levels of viral burden in the nose and lungs of mice infected with Omicron were 10–100-fold lower than in those infected with Beta, three days after infection.

In experiments involving Syrian hamsters and transgenic hamsters expressing the human ACE2 receptor, Omicron infection also resulted in no weight loss. Lung infection, clinical disease and pathology with Omicron were also milder compared to other variants of concern, including Delta.

The authors note that experiments are ongoing to determine the underlying mechanisms that result in the reduced levels of infection seen in rodent models and to determine how this relates to Omicron infection in humans.