Background: Solid organ transplant recipients (SOTRs) are at high risk for severe coronavirus disease 2019 (COVID-19) due to comorbidities and immunosuppression. Moreover, the treatment for COVID-19 is limited. Neutralizing monoclonal antibodies (mAbs) against severe acute respiratory syndrome coronavirus 2 are used for treating mild-to-moderate COVID-19. However, reports comparing the efficacy of COVID-19 treatment with and without neutralizing mAbs in SOTRs are limited. Here, we assessed the efficacy of casirivimab/imdevimab as neutralizing mAbs against mild-to-moderate COVID-19 in SOTRs. Methods: Forty-one patients hospitalized from September 2020 to September 2021 at Tokyo Women’s Medical University Hospital were included. The duration until antifebrile and of hospitalization, requirement of oxygen (O2) ≥ 5 L, and neutralizing antibody levels were compared in SOTRs with COVID-19 who received and those who did not receive casirivimab/imdevimab therapy.Results: Casirivimab/imdevimab therapy correlated with less duration until antifebrile and of hospitalization and non-requirement of O2 ≥ 5 L in SOTRs with COVID-19 [mean days without vs. with casirivimab/imdevimab: 6 vs. 2; P = 0.0002, hazard ratio (HR) = 0.3333, 95% confidence interval (CI) = 0.1763–0.6301; mean days: 15 vs. 8; P < 0.0001, HR = 0.5333, 95% CI = 0.2878–0.9883; P = 0.0377, HR = 0.1502, 95% CI = 0.02511–0.8980, respectively]. Further, casirivimab/imdevimab therapy was associated with early antifebrile after adjusting for sex and age (P = 0.013, HR = 0.412, 95% CI = 0.205–0.826). Although no significant difference was observed in neutralizing antibody levels between patients without and with casirivimab/imdevimab therapy on the day of hospitalization (P = 0.1055), including 13 SOTRs who had received vaccination. Neutralizing antibody levels were higher in patients with casirivimab/imdevimab therapy at 3–5 days after hospitalization including such patients in whom the levels tended to stay lower despite vaccination than in those without casirivimab/imdevimab at 7–9 days after hospitalization (P < 0.0001, mean antibody without vs. with, 414.9 vs. 40000). Conclusion: Casirivimab/imdevimab therapy was effective and increased the neutralizing antibody levels in SOTRs with mild-to-moderate COVID-19. Therefore, casirivimab/imdevimab therapy could contribute to preventing the progression of mild-to-moderate COVID-19 in SOTRs.
