It means that in the study submitted to the FDA, the vaccine group had 8 cases and the control group had 162 cases [0]. 8 / 162 = ~5% of the cases in the vaccin

It means that in the study submitted to the FDA, the vaccine group had 8 cases and the control group had 162 cases [0]. 8 / 162 = ~5% of the cases in the vaccine group, ergo 95% effectiveness (or more specifically Relative Risk Reduction). [0] https://www.nejm.org/doi/full/10.1056/NEJMoa2034577 https://www.nejm.org/doi/full/10.1056/NEJMoa2034577