Subcutaneous REGEN-COV Antibody Combination to Prevent Covid-19

19To the Editor: O'Brien et al. report that the combination of casirivimab and imdevimab (REGEN-COV) reduced the risk of symptomatic Covid-19 by 81.4% among persons exposed to SARS-CoV-2. 1 However, we question the decision to communicate the effect size almost exclusively with the use of the relative risk reduction, calculated as 1 minus the relative risk.The authors placed a strong emphasis on the relative risk reduction for both primary and secondary efficacy end points throughout the abstract and main text but did not provide a complementary, absolute measure.The Consolidated Standards of Reporting Trials guidelines recommend reporting both the absolute risk reduction and the relative risk reduction to fully describe the effectiveness of an intervention, as applied in many trials, including the recent trial of bamlanivimab-etesevimab combination for This practice is imperative, because the same relative risk reduction may represent very different absolute risk reductions -a relative risk reduction of 80% corresponds with an absolute risk reduction of 40% when the absolute risk decreases from 50% to 10% but with an absolute risk reduction of less than 1% when the absolute risk decreases from 1% to 0.2%.The provision of only the relative risk measure may lead clinicians, patients, and laypeople to overestimate treatment efficacy.