Determinants of passive antibody effectiveness in SARS-CoV-2 infection

Neutralising antibodies are an important correlate of protection from SARS-CoV-2 infection. Multiple studies have investigated the effectiveness of passively administered antibodies (either monoclonal antibodies, convalescent plasma or hyperimmune immunoglobulin) in preventing acquisition of or improving the outcome of infection. Comparing the results between studies is challenging due to different study characteristics including disease stage, trial enrolment and outcome criteria, and different product factors, including administration of polyclonal or monoclonal antibody, and antibody targets and doses. Here we integrate data from 37 randomised controlled trials to investigate how the timing and dose of passive antibodies predicts protection from SARS-CoV-2 infection. We find that both prophylactic and early therapeutic administration (to symptomatic ambulant subjects) have significant efficacy in preventing infection or progression to hospitalisation respectively. However, we find that effectiveness of passive antibody therapy in preventing clinical progression is significantly reduced with administration at later clinical stages (p<0.0001). To compare the dose-response relationship between different treatments, we normalise the administered antibody dose to the predicted neutralisation titre (after dilution) compared to the mean titre observed in early convalescent subjects. We use a logistic model to analyse the dose-response curve of passive antibody administration in preventing progression from symptomatic infection to hospitalisation. We estimate a maximal protection from progression to hospitalisation of 70.2% (95% CI: 62.1 - 78.3%). The dose required to achieve 50% of the maximal effect (EC-50) for prevention of progression to hospitalisation was 0.19-fold (95% CI: 0.087 - 0.395) of the mean early convalescent titre. This suggests that for current monoclonal antibody regimes, doses between 7- and >1000-fold lower than currently used could still achieve around 90% of the current effectiveness (depending on the variant) and allow much more widespread use at lower cost. For convalescent plasma, most current doses are lower than required for high levels of protection. This work provides a framework for the rational design of future passive antibody prophylaxis and treatment strategies for COVID-19.