Abstract: Received: 21 October 2021 | Revised: 7 December 2021 | Accepted: 9 December 2021 DOI: 10.1002/jgf2.516 ORIGINAL ARTICLE The effect of casirivimab with imdevimab on disease progression in nonsevere COVID-­19 patients in a single hospital in Japan Junpei Komagamine MD Department of Internal Medicine, National Hospital Organization Tochigi Medical Center, Utsunomiya, Japan Correspondence Junpei Komagamine, Department of Internal Medicine, National Hospital Organization Tochigi Medical Center, 1-­10-­37, Nakatomatsuri, Utsunomiya, Tochigi, Japan. Email: junpei0919@yahoo.co.jp | Taku Yabuki MD | Satsuki Yoshihara MD | Nao Tanaka MD Abstract Background: Recent randomized trials have revealed that neutralizing monoclonal antibodies can reduce disease progression in mild–­moderate COVID-­19 patients. However, no studies have investigated the effect of neutralizing monoclonal antibodies on clinical outcomes in Japan. Methods: A single-­center retrospective and prospective cohort study was conducted. All consecutive febrile nonsevere COVID-­19 patients with at least one risk factor were included. The primary outcome was progression to severe COVID-­19. Severe COVID-­19 cases were defined as patients requiring oxygen therapy and dexamethasone. The differences in the primary outcomes between the patients who were treated with casirivimab with imdevimab (treatment group) and those who were not (control group) were compared using the chi-­squared test. Results: A total of 128 patients were included. Of those, the mean age was 53.6 years old (SD 9.9), and 52 (40.6%) were women. Fifty-­three patients were treated with casirivimab with imdevimab, and 75 patients were given the standard treatment only. The primary outcome occurred in eight (15.1%) of the 53 patients in the treatment group and 33 (44.0%) of the 75 patients in the control group (odd ratio [OR] 0.23, 95% CI 0.09 to 0.55). The multivariate analysis revealed that the use of casirivimab with imdevimab (OR 0.21, 95% CI 0.08 to 0.54) was the only independent risk factor associated with progression to severe COVID-­19. No patients died during hospitalization in either group. Conclusion: Similar to other countries, casirivimab with imdevimab significantly reduced disease progression in early nonsevere COVID-­19 patients with fever and risk factors in Japan. KEYWORDS casirivimab, COVID-­19, imdevimab This is an open access article under the terms of the Creat​ive Commo​ns Attri​bution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2021 The Authors. Journal of General and Family Medicine published by John Wiley & Sons Australia, Ltd on behalf of Japan Primary Care Association. J Gen Fam Med. 2021;00:1–6.  wileyonlinelibrary.com/journal/jgf2 | 1 2 1 | KOMAGAMINE et al. | I NTRO D U C TI O N the institutional Medical Ethics Committee because we collected de-­identified data without contacting the patients. However, per Since the first case of severe acute respiratory syndrome coronavi- the Japanese Ethical Guidelines, we displayed an opt-­o ut state- rus 2 (SARS-­CoV-­2) was documented in December 2019 in China,1 ment in the waiting room and on the webpage of the hospital to several treatments for coronavirus disease 2019 (COVID-­19) have inform patients about the study and to provide the opportunity been developed. Dexamethasone and interleukin (IL)-­6 receptor for patients to decline the use of..