Breakthrough COVID-19 and casirivimab-imdevimab treatment during a SARS-CoV-2 B1.617.2 (Delta) surge

Introduction:: The impact of vaccination and casirivimab-imdevimab monoclonal antibody treatment on the clinical outcome of COVID-19 during a period of SARS-CoV-2 Delta surge is not known. Aim and Methods:: All patients with COVID-19 at our facilities in the US Midwest were enrolled to assess breakthrough cases among vaccinated individuals and to compare the rates of hospitalization between casirivimab-imdevimab treated versus untreated patients. The study period occurred in July 2021 during a period dominated by the Delta variant. Results:: The majority (68.1%) of 630 COVID-19 cases occurred in unvaccinated individuals. Among 403 patients eligible for monoclonal antibody treatment, the 28-day hospitalization rate was 2.6% of 112 patients who received treatment with casirivimab-imdevimab, compared to 16.6% of 291 eligible high-risk patients who did not receive casirivimab-imdevimab (Odds Ratio [OR]: 0.138, 95% confidence interval (CI): 0.0426–0.4477, p = 0.001). Casirivimab-imdevimab treatment was associated with lower rates of hospitalization among the vaccinated and unvaccinated cohorts. Conclusions:: During a SARS-CoV-2 Delta surge, breakthrough COVID-19 occurred among vaccinated persons, especially among those with multiple medical comorbidities. Casirivimab-imdevimab treatment was associated with significantly lower rates of hospitalization in vaccinated and unvaccinated persons.