A randomized phase 2a trial found that monoclonal antibody AER002 is safe but ineffective for Long COVID overall. Investigators reported that post-hoc analyses pointed to potential benefit in a subset of patients with low baseline anti-spike antibodies and higher drug exposure, but the headline result did not support broad efficacy. The study adds to the growing set of setbacks in Long COVID biologics development, where heterogeneous patient biology and timing of intervention often complicate trial design. For developers, the findings reinforce the need for better patient selection and exposure-response characterization early. Clinically, the result may influence how companies frame future trials around stratification biomarkers and mechanistic hypotheses, particularly when antibodies target a single immunologic axis and Long COVID likely spans multiple causal pathways.