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

Fu, W.; Li, J.; Scheet, P.

Background Two 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. Methods We examined data in both vaccine trials, provided 95% confidence intervals, and projected the cases that would be prevented in communities of multi-million population. Results The 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. Conclusions Point 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. Key words: Confidence interval, Exposure, Point estimate, Poisson distribution, Risk ratio.