A 2021 observational study assessed the sensitivity of the SARS-CoV-2 Delta variant (B1.617.2) to monoclonal antibodies that target the receptor-binding domain (RBD) including bamlanivimab (LY-CoV555) and etesevimab (LY-CoV016). A strain of the Delta variant was isolated from a patient with COVID-19 who had traveled to India and returned to France. The neutralization of the Delta variant by bamlanivimab and etesevimab was presented in graphs. Bamlanivimab had low antiviral activity versus the Delta variant at concentrations between 0.001 µg/mL and 0.1 µg/mL, which may be caused by an RBD mutation, L452R, acting as an escape mutation for this antibody. The activity of bamlanivimab increased between concentrations of 0.1 µg/mL and 10 µg/mL, but was still relatively low. Etesevimab had higher activity against the Delta variant compared to bamlanivimab between concentrations of 0.001 µg/mL and 10 µg/mL. Increasing activity of etesevimab against the Delta variant occurred between 0.001 µg/mL and 0.01 µg/mL. Little change in activity occurred after 0.01 µg/mL for etesevimab. Additionally, the binding of bamlanivimab to Vero cells infected with the Delta variant was evaluated using flow cytometry. Bamlanivimab did not bind to the Delta variant in this experiment, which may explain the escape of the Delta variant from neutralization by certain antibodies. 
A 2021 observational study evaluated whether various neutralizing antibodies are able to inhibit host cell entry driven by the S protein of the SARS-CoV-2 Delta variant (B.1.617). Bamlanivimab did not block entry driven by the S protein, which is due to bamlanivimab resistance mutations found in the epitope recognized by the antibody. However, etesevimab resulted in blocked entry driven by the S protein. Combination of bamlanivimab and etesevimab did not result in effective inhibition of the S protein-driven cell entry. These results show bamlanivimab monotherapy may not be effective for treatment of patients with the Delta variant. 
The production of bamlanivimab/etesevimab has been halted by the manufacturer until further notice. Furthermore, the combination product is not recommended for use. This is due to in vitro assays indicating that bamlanivimab and etesevimab together are not active against P.1/Gamma and B.1.315/Beta variants of COVID-19 which are trending upwards in the United States (exceeding 11% at the time of this publication).