A statistician explains: What does ‘90% efficacy’ for a Covi
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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.

Atanu Biswas is Professor of statistics at the Indian Statistical Institute, Kolkata explains 'Vaccine efficacy'.

'Vaccine efficacy' is expressed as a proportionate reduction in disease attack rate (AR) between the unvaccinated (ARU) and vaccinated (ARV) groups under the phase III trial.

ARV is just the proportion of individuals within the vaccinated group who got infected within the study period. Similarly, ARU is the proportion of infected within the unvaccinated group.

The ratio of ARV to ARU is called the risk ratio, RR. A lower value of RR clearly indicates better performance of the potential vaccine.

When both the vaccinated and unvaccinated groups have, more or less, equal number of individuals, RR is the simple ratio of the number of infected in the vaccine group to that in the placebo group. And, one minus RR, expressed in percentage, is called the “Vaccine Efficacy”.

Clearly, higher the value of vaccine efficacy, the better the performance of the potential vaccine. Thus, a 90% efficacy (ie 10% RR) means that the proportion of infection in the vaccinated group is about one-tenth of the proportion of infection in the placebo group.

Example: The Moderna trial involved 30,000 people - approximately half receiving the vaccine, and the remaining a “placebo”, in each trial.

Of the first 95 to develop Covid-19 symptoms in the Moderna trial, only five were in the vaccine group, and the remaining 90 were in the placebo group.

Thus, the RR for Moderna vaccine up to this point of time is 5/90, and vaccine efficacy is (1 – 5/90), which is 94.44%.

Short-term data: As per the 2020 ‘Guidance for Industry’ of the US Food and Drug Administration for ‘Development and Licensure of Vaccines to Prevent COVID-19’, all phase III trial participants are expected to be monitored for at least 1 year.

The recruitment of phase III for the Moderna vaccine was done in April and May, and that for Pfizer vaccine began in end-July. Thus, these all are short-term efficacy.

Source: https://scroll.in/article/979627/a-statistician-explains-what-does-90-efficacy-for-a-covid-19-vaccine-mean
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