While I was writing a newspaper article on performances of various potential Covid-19 vaccines from different organisations, my wife, who is neither a statistician nor a vaccine expert, asked me the meaning of 90% efficacy of a vaccine. And I suddenly realised that such figures for potential vaccines under trial are never very clear to common people unless their implications are clearly illustrated.

In fact, different pharmaceutical companies are now coming out with the success stories of their potential vaccines, and the efficacy rates of these vaccines are reported to be high – 70%, 90%, 95%, and so on.

Here is how we can understand vaccine efficacy.

Phase III trial

Vaccines are usually approved on the basis of results from three stages of clinical trials. The trials aim to assess short-term safety, ability to generate an immune response, and efficacy. Short-term safety is usually judged in early phases.

Phase III trial, which is often the most elaborate one, investigates the efficacy of the vaccine under trial – usually in comparison to a placebo, which maybe a similar-looking injection having no medical effect. In a phase III trial, often thousands of people are given the vaccine or a placebo, and then these people are monitored over several...

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