Covid-19 Vaccine Efficacy: Accuracy, Uncertainty and Projection of Cases

BackgroundTwo vaccine candidates for coronavirus disease 2019 (Covid-19) have been announced by Pfizer-BioNTech and Moderna with above 90% efficacy. The efficacy of each vaccine changes between reports with no accuracy assessment. MethodsWe examined data in both vaccine trials, provided 95% confidence intervals, and projected the cases that would be prevented in communities of multi-million population. ResultsThe 95% confidence intervals reveal that the true vaccine efficacy could be as low as 86% for stated efficacy of 94.4% in an interim report, indicating the inaccuracy and uncertainty of efficacy point estimate. Both vaccines achieve an efficacy above 89% by the 95% confidence interval in updated reports. The Moderna vaccine would prevent more than 50,260 cases in communities of 1 million people with 1 year exposure. ConclusionsPoint estimates of vaccine efficacy transmit limited information. Corresponding statements of uncertainty, such as confidence intervals, should be provided and included in discussions of societal impact. The Covid-19 vaccines announced to date would prevent a substantial number of cases even at lower ends of the intervals.