The news that the Pfizer-BioNTech and Moderna vaccines are 95 percent effective at preventing COVID-19 disease, and the AstraZeneca-Oxford vaccine might be 90 percent effective (with the right dosage), has given the world a much-needed glimmer of hope. These observed efficacy rates of the vaccines from their Phase 3 clinical trials puts them in the range of the most valuable vaccines in use today and imply that the pandemic would be tamed rapidly when sufficient numbers of people received them. They also substantially exceed what was expected by the trial sponsors and public health officials.
While this is certainly welcome news, it requires context, based on the statistical methods used to evaluate efficacy in clinical trials. In the case of the Pfizer-BioNTech vaccine, for example, the estimated efficacy is based on 170 observed cases of COVID-19 among trial participants, 162 of whom received the placebo. Thus, only 8 trial volunteers who received the vaccine developed COVID-19, which translates into an observed efficacy rate of about 95 percent.
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