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The novel coronavirus disease COVID-19 has become one of the most socially significant infections. One of the main models for COVID-19 pathogenesis study and anti-COVID-19 drug development is laboratory animals sensitive to the virus. Herein, we report SARS-CoV-2 infection in novel transgenic mice conditionally expressing human ACE2 (hACE2), with a focus on viral distribution after intranasal inoculation. Transgenic mice carrying hACE2 under the floxed STOP cassette [(hACE2-LoxP(STOP)] were mated with two types of Cre-ERT2 strains (UBC-Cre and Rosa-Cre). The resulting offspring with temporal control of transgene expression were treated with tamoxifen to induce the removal of the floxed STOP cassette, which prevented hACE2 expression. Before and after intranasal inoculation, the mice were weighed and clinically examined. On Days 5 and 10, the mice were sacrificed for isolation of internal organs and the further assessment of SARS-CoV-2 distribution. Intranasal SARS-CoV-2 inoculation in hACE2-LoxP(STOP)×UBC-Cre offspring resulted in weight loss and death in 6 out of 8 mice. Immunostaining and focus formation assays revealed the most significant viral load in the lung, brain, heart and intestine samples. In contrast, hACE2-LoxP(STOP) × Rosa-Cre offspring easily tolerated the infection, and SARS-CoV-2 was detected only in the brain and lungs, whereas other studied tissues had null or negligible levels of the virus. Histological examination revealed severe alterations in the lungs, and mild changes were observed in the brain tissues. Notably, no changes were observed in mice without tamoxifen treatment. Thus, this novel murine model with the Cre-dependent activation of hACE2 provides a useful and safe tool for COVID-19 studies.

The circulation of seasonal influenza in 2020–2021 around the world was drastically reduced after the start of the COVID-19 pandemic and the implementation of mitigation strategies. The influenza virus circulation reemerged in 2021–2022 with the global spread of the new genetic clade 3C.2a1b.2a.2 of A(H3N2) viruses. The purpose of this study was to characterize influenza viruses in the 2021–2022 season in Russia and to analyze the receptor specificity properties of the 3C.2a1b.2a.2 A(H3N2) viruses. Clinical influenza samples were collected at the local Sanitary-and-Epidemiological Centers of Rospotrebnadzor. Whole genome sequencing was performed using NGS. The receptor specificity of hemagglutinin was evaluated using molecular modeling and bio-layer interferometry. Clinical samples from 854 cases of influenza A and B were studied; A(H3N2) viruses were in the majority of the samples. All genetically studied A(H3N2) viruses belonged to the new genetic clade 3C.2a1b.2a.2. Molecular modeling analysis suggested a higher affinity of hemagglutinin of 3C.2a1b.2a.2. A(H3N2) viruses to the α2,6 human receptor. In vitro analysis using a trisaccharide 6’-Sialyl-N-acetyllactosamine receptor analog did not resolve the differences in the receptor specificity of 3C.2a1b.2a.2 clade viruses from viruses belonging to the 3C.2a1b.2a.1 clade. Further investigation of the A(H3N2) viruses is required for the evaluation of their possible adaptive advantages. Constant monitoring and characterization of influenza are critical for epidemiological analysis.