Abstract
Although the coronavirus disease 2019 (COVID-19) pandemic is coming to an end, it still poses a threat to the immunocompromised and others with underlying diseases. Especially in cases of persistent COVID-19, new mutations conferring resistance to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) therapies have considerable clinical implications. We present a patient who independently acquired a T21I mutation in the 3CL protease after nirmatrelvir exposure. The T21I mutation in the 3CL protease is one of the most frequent mutations responsible for nirmatrelvir resistance. However, limited reports exist on actual cases of SARS-CoV-2 with T21I and other mutations in the 3CL protease. The patient, a 55 year-old male, had COVID-19 during chemotherapy for multiple myeloma. He was treated with nirmatrelvir early in the course of the disease but relapsed, and SARS-CoV-2 with a T21I mutation in the 3CL protease was detected in nasopharyngeal swab fluid. The patient had temporary respiratory failure but later recovered well. During treatment with remdesivir and dexamethasone, viruses with the T21I mutation in the 3CL protease showed a decreasing trend during disease progression while increasing during improvement. The impact of drug-resistant SARS-CoV-2 on the clinical course, including its severity, remains unknown. Our study is important for examining the clinical impact of nirmatrelvir resistance in COVID-19.