Friday December 18, 2020
The Food and Drug Administration (FDA) confirmed that the Pfizer covid vaccine is 95 percent effective, but what does that statement mean? The details of the Pfizer covid vaccine trial can be found here. As was the case in the previous announcement about the vaccine, this “efficacy” figure is misleading. The incidence of covid “cases” in the placebo group was 162 out of 18,325 subjects, or less than 1 percent. The incidence of “cases” in the vaccine group was 8 out of 18,198, or about one-twentieth the incidence of “cases” in the placebo group. The “efficacy” of 95 percent is an odds ratio between groups, but this ratio is comparing two uncommon or rare events. Although the benefit was statistically significant, the number to vaccinate in order to prevent a single “case” was 122, which is not nearly so impressive. For severe cases, the result was not statistically significant due to very small numbers (one severe case in the vaccine group and three severe cases in the control group). Even if these percentages persisted to statistical significance with larger numbers of subjects, the number to vaccinate in order to prevent a single severe case was over nine thousand, which is not impressive at all. There has been no statistically significant mortality benefit to date, and none of the deaths in either group (two in the vaccine group and four in the control group) were associated with covid-19.
Details of Trial
This was a double-blinded randomized control trial with 21,823 subjects in the vaccine group and another 21,828 subjects in the placebo group. Double-blind trials mean that neither the subjects nor the caregivers know whether subjects received vaccine or placebo. Randomized means that the subjects were put into the vaccine or placebo group by chance. The randomization is performed in a manner to balance the vaccine and placebo groups on the basis of significant characteristics such as age, race, and comorbid medical conditions. Subjects could not choose the group into which they would be enrolled.