Real-World Effectiveness of Casirivimab and Imdevimab Among Patients Diagnosed With COVID-19 in the Ambulatory Setting: A Retrospective Cohort Study Using a Large Claims Database

ImportanceData on real-world effectiveness of casirivimab and imdevimab (CAS+IMD) for treatment of coronavirus 2019 (COVID-19) are limited, especially with regard to variants of concern such as Delta.ObjectiveTo assess effectiveness of CAS+IMD versus no COVID-19 antibody treatment among patients diagnosed with COVID-19 in the ambulatory setting overall and among subgroups, including patients diagnosed during the Delta-dominant period prior to Omicron emergence.DesignRetrospective cohort study.SettingKomodo Health closed claims database.ParticipantsPatients diagnosed with COVID-19 in ambulatory settings from December 2020 through September 2021 treated with CAS+IMD or untreated but treatment-eligible under the Emergency Use Authorization (EUA). Each treated patient was exact- and propensity score-matched without replacement to up to 5 untreated EUA-eligible patients.ExposureCAS+IMD treatment.Main Outcomes and MeasuresComposite endpoint of 30-day all-cause mortality or COVID-19-related hospitalization. Kaplan-Meier estimators were used to calculate risk of outcome overall and across subgroups defined by age groups, COVID-19 vaccination status, immunocompromised, and timing of COVID-19 diagnosis (December 2020 to June 2021, and July 2021 to September 2021). Cox proportional-hazards models were used to estimate adjusted hazard ratios (aHR) and 95% confidence intervals (95% CI).ResultsAmong 75 159 patients treated with CAS+IMD and 1 670 338 EUA-eligible untreated patients, 73 759 treated patients were matched to 310 688 untreated patients; matched patients had an average age ∼50 years, approximately 60% were women and were generally well-balanced across risk factors. The 30-day risk of the composite outcome was 2.1% and 5.2% in the CAS+IMD -treated and untreated patients, respectively; CAS+IMD treatment was associated with a 60% lower risk of the outcome (aHR 0.40; 95% CI, 0.38-0.42). The effect of CAS+IMD treatment was consistent across subgroups, including those who received a COVID-19 vaccine (aHR 0.48, 95% CI, 0.41-0.56), and those diagnosed during the Delta-dominant period (aHR 0.40, 95% CI, 0.38-0.42).Conclusions and RelevanceThe real-world effectiveness of CAS+IMD is consistent with the efficacy for reducing all-cause mortality or COVID-19-related hospitalization reported in clinical trials. Effectiveness is maintained across patient subgroups, including those who may be prone to breakthrough infections, and was effective against susceptible variants including Delta.□