The authors then infected different sets of transgenic mice designed to be susceptible to SARS-CoV-2 with one of the three strains: Omicron, the ancestral virus, or the Omi-S hybrid virus (here).
In the study, 100% of mice infected with the ancestral virus strain died, while 80% of those infected with Omi-S died, and none of the mice infected with Omicron died.
Others shared the claim that the Omi-S had an “80% kill rate,” without specifying whether this referred to mice or humans.
The ancestral virus (the viruses as they were when introduced in Europe and the US) in this mouse model kills 100% of the animals.
Early in the pandemic, the fatality rate of the ancestral virus for people in a multicountry European analysis was estimated at about 4% (here).