Syrian hamsters (male, 4 weeks old) were purchased from Japan SLC and divided into groups by simple randomization. Baseline body weights were measured before infection. For the virus infection experiments, hamsters were anaesthetized by intramuscular injection of a mixture of 0.15 mg kg−1 medetomidine hydrochloride (Domitor, Nippon Zenyaku Kogyo), 2.0 mg kg−1 midazolam (Dormicum, FUJIFILM Wako Chemicals) and 2.5 mg kg−1 butorphanol (Vetorphale, Meiji Seika Pharma). The B.1.1 virus, Delta, Omicron (10,000 TCID50 in 100 µl) or saline (100 µl) were intranasally inoculated under anaesthesia. Oral swabs were daily collected under anaesthesia with isoflurane (Sumitomo Dainippon Pharma). Body weight, enhanced pause (Penh, see below), the ratio of time to peak expiratory follow relative to the total expiratory time (Rpef, see below) and subcutaneous oxygen saturation (SpO2, see below) were monitored at 1, 3, 5, 7, 10, and 15 d.p.i. Respiratory organs were anatomically collected at 1, 3, 5 and 7 d.p.i. (for lung) or 1, 3 and 7 d.p.i. (for trachea). Viral RNA load in the oral swabs and respiratory tissues was determined by RT–qPCR. Viral titres in the lung hilum were determined by TCID50. These tissues were also used for histopathological and IHC analyses (see below). No method of randomization was used to determine how the animals were allocated to the experimental groups and processed in this study because covariates (sex and age) were identical. The number of investigators was limited, as most of experiments were performed in high-containment laboratories. Therefore, blinding was not carried out.
Suzuki R., Yamasoba D., Kimura I., Wang L., Kishimoto M., Ito J., Morioka Y., Nao N., Nasser H., Uriu K., Kosugi Y., Tsuda M., Orba Y., Sasaki M., Shimizu R., Kawabata R., Yoshimatsu K., Asakura H., Nagashima M., Sadamasu K., Yoshimura K., Sawa H., Ikeda T., Irie T., Matsuno K., Tanaka S., Fukuhara T, & Sato K. (2022). Attenuated fusogenicity and pathogenicity of SARS-CoV-2 Omicron variant. Nature, 603(7902), 700-705.