Discussion
SARS
Although the mRNA-1273 vaccine and casirivimab + imdevimab mAbs for COVID-19 prevention have high efficacy over the duration studied, the relative contribution of antibody at the time of exposure differs. For nAb titers greater than 1000 IU50/ml, the clinical efficacy of both mRNA-1273 and casirivimab + imdevimab to prevent COVID-19 is >90%, with extant antibody being responsible for approximately 72% of the total vaccine effect at a nAb titer of 1000 IU50/ml. At lower titers (e.g., <100 IU50/ml), mRNA-1273 efficacy persists, in contrast to the waning efficacy associated with the mAb combination casirivimab + imdevimab, where precise quantitation of protective efficacy at lower nAb titers was not possible due to the low number of COVID-19 cases at late timepoints in COV-2069.While the efficacy of casirivimab + imdevimab to prevent SARS CoV-2 is driven entirely by passive immunity with exogenous monoclonal antibodies