A Bayesian interpretation of Pfizer vaccine efficacy

Pfizer has just announced that their vaccine trial had concluded with an impressive result: 95% efficacy [1]. One does not need to do any fancy analysis to see that the efficacy is real. Here, I would like to offer a Bayesian interpretation of the efficacy.

Briefly, the trial has enrolled 43,661 volunteers, and as of 13/11/2020 41135 volunteers have received a second dose of the vaccine. Assuming that half the 41135 participants have been randomly assigned to the vaccine group, and the rest to placebo. According to the press release, 170…