In a limited number of cases when a full virus does not contain any known gain of function mutations, such virus may be engineered to enable the assessment of antiviral activity in cells. In addition, in vitro resistance selection experiments are undertaken in cells incubated with SARS-CoV-2 and nirmatrelvir in our secure Biosafety level 3 (BSL3) laboratory to assess whether the main protease can mutate to yield resistant strains of the virus.
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