How is the efficacy of a vaccine measured?
A vaccine's efficacy rate is calculated in large clinical trials and it's very complicated, but we could simply separate these tested people into two groups in this presentation. Half of them receive the vaccine and half get a placebo. They go back to living their normal lives for a period of time while scientists track their health conditions.
For example, in the trial for Sinovac up to December 16, 2020, there were 12,396 participants. In the end, 253 people were infected with COVID-19.
If the 253 were evenly split, that would mean people are just as likely to get sick with the vaccine as without it. Therefore, it would have a 0% efficacy.
If all 253 were in the placebo group and zero people who received the vaccine became sick, the vaccine would have an efficacy of 100%.
With this particular trial, there were 168 test subjects in the placebo group and 85 in the vaccine group. The vaccine had 50.7% efficacy results.