In Reply We appreciate the concerns expressed by Dr Stone and colleagues that the commercially available assay for antibodies against the SARS-CoV-2 RBD used in our Research Letter measures total immunoglobulin levels rather than IgG levels. That said, this assay, particularly given its wide availability, has played a critical role in research about vaccines and convalescent immunity. For example, our work using this assay showed poor vaccine immunogenicity in populations at high risk of breakthrough infections, and improvement with additional vaccine doses, which led to changes in treatment guidance for immunocompromised patients by the CDC and US Food and Drug Administration.
