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Expert Decodes What Vaccine Efficacy is And Why It Can Differ from One Trial to Another

File photo of Covaxin vaccine.

File photo of Covaxin vaccine.

Vaccine efficacy is determined in randomised-control trials, in which volunteers receive either a vaccine or placebo.

Efficacy (of vaccines) is basically percentage reduction against a specified outcome. That outcome is predetermined. Also, in a clinical trial, you can fix multiple outcomes. For example, the outcome against clinical disease, which means any form of the disease, mild, moderate or severe. Added to that, the outcome against asymptomatic cases, etc. So you select a particular outcome and against it, efficacy is calculated.

Vaccine efficacy is determined in randomised-control trials, in which volunteers receive either a vaccine or placebo. These trials tell researchers how well the vaccine works in optimal conditions, in which its storage and delivery are monitored and volunteers are healthy. To calculate efficacy rates, scientists compare the frequency of covid-19 infection in the vaccinated and the placebo groups.

So efficacy is against the total number of cases that occur. what proportion was reduced? Therefore, if the data says that there is 78 per cent efficacy, it means that the probability of a person contracting that disease after being vaccinated is reduced by 78 per cent.

Therefore, you subtract the cases that you get in the trial arm that were infected and divide it by the total number of cases and that is how you calculate efficacy. Also, you must have seen that the data says the vaccine has 100 per cent efficacy against severe disease. It means that likelihood of being hospitalised and being subjected to very high levels of infections is reduced by 100 per cent.

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And similarly, they have another parameter, the risk of asymptomatic infection which is 70 per cent. It essentially means the same thing that people who were tested positive but had no symptoms, the chances of falling in that category is reduced by 70 per cent.

Also, efficacy numbers may differ from one trial to another because the trial is designed in a particular way. These trials are done on thousands of people. They have certain data points in place and because each trial has a different set of data and sample size, the efficacy numbers differ to a certain extent. But between two results of clinical trials, there is something called the confidence interval which essentially means that analysis has been carried in two different environments. The lower the confidence interval the better it is.

So this is a normal phenomenon that there is minor variation in the number. It happens to all the trials and all the settings for additional data they have identified.

Added to that is the fact that, the vaccine is tested on the available strain of the virus. It is not known what specific strain is being tested. Specific mutant testing of the vaccine is done separately in labs. Therefore, the current COVXIN’s efficacy is against the virus which is currently spreading.

(As told to Rounak Kumar Gunjan)

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first published:April 22, 2021, 09:16 IST