TBC Explainer: What does the efficacy of the Covid-19 vaccine tell us

No, efficacy rate does not signify that one Covid-19 vaccine is better than the other.
All you need to know about Covid-19 vaccine efficacy
All you need to know about Covid-19 vaccine efficacyImage: The Bridge Chronicle

While India is conducting vaccination drives, there has been a sentiment of apprehension about the vaccine. The uncertainty comes as a result of the emergency approvals given to the vaccine. Additionally, wit the emergence of variants of the virus, the need for a vaccine becomes urgent. And while vaccination drives continue in India, there is a need to gain all possible knowledge about vaccines available.

Last week Bharat Biotech revealed the efficacy rate of the vaccine — Covaxin. The announcement came after the Phase III trials of the vaccine, which was 81 per cent. Oxford-AstraZeneca also released the efficacy rate, which stands under the wide range of Covishield 62-90 per cent. But how do we understand these efficacy rates?

The vaccine efficacy rate does not signify that one vaccine is better than the other. It also needs to be considered that efficacy can differ according to regions. The Bridge Chronicle explains the concept behind the vaccine efficacy rate.

A common understanding of the efficacy rate is that if a vaccine has a 95 per cent efficacy, you have a 5 per cent chance of getting sick even if you get vaccinated. Many publications have also reported on similar lines. But it is important to understand efficacy for what it is. Jim Daley quoted Namandjé Bumpus, a pharmacologist at Johns Hopkins Medicine in Baltimore, and Ashley Lauren St. John, an immunologist at Duke-NUS Medical School in Singapore in Scientific American saying, "Efficacy is calculated based on trials that have an unvaccinated placebo control group, and at the end of the trial, they look at the number in the control group that ended up with symptomatic COVID to get the baseline infection rate."

Factors affecting efficacy

Apart from this, multiple factors are affecting the efficacy rate of the vaccine. The associated factors are complicated statistical measures that raise or lower the efficacy levels.

For instance, in some cases, only symptomatic people were considered for efficacy testing. So, vaccine producers could assess how much their vaccines reduce the risk of developing symptoms. But, the risk reduction in asymptomatic people was uncertain. However, asymptomatic people were (assumed to be) responsible for the rapid spread of the virus, and hence this information becomes vital.

Additionally, the rate also shows where the vaccine stands according to the World Health Organisation standards, currently at 50 per cent. Meaning the vaccine should be able to lower the risk of contracting Covid-19 by 50 per cent. Considering this, it becomes difficult to judge a single vaccine based only on the efficacy rate.

However, the efficacy rate is the only statistic available to the general public. And when deciding on a vaccine, it becomes essential to understand the meaning of efficacy rate.

Changing efficacy rates

The efficacy rate of the vaccine can often change when considering different parameters. Take AstraZeneca as an example. The efficacy rates of the vaccine changes among the same group of people.

According to a report on the official website, in the Phase III trials, (when) two full doses were administered 12 weeks apart, the efficacy rate was 82 per cent. But within the group, some people below the age of fifty-five were given the dosage differently. The group received a half-dose, followed by a full dose and the efficacy rate rose to 90 per cent, as reported by The Wire.

The Bridge Chronicle also spoke to Dr Sanjay Patil, the chairman of the Hospital Board of India to understand efficacy better.

Explaining the concept of efficacy, the doctor said, "After the trials, when a vaccine gets approval, the efficacy of the vaccine is decided. If we consider the efficacy to be at 70 per cent. It means that 70 per cent of them are safe. Eg. Out of 100 people who were vaccinated, 70 are safe."

Should the efficacy rate decide which vaccine you take? "No. Since we don't have a treatment for covid right now, it is important to take a vaccine even if the efficacy is 50-60 per cent. And currently the efficacy is higher, so it is good," explains Dr Patil.

Why do different vaccines show different efficacy rates? The doctor explains, "For every vaccine, the control group and study group is different. So the number of volunteers is different. Along with that the time duration also differes. All of this affects efficacy."

Does the efficacy rate signify that I will be protected from COVID-19? "Yes it does."

Having said that, the efficacy of the Covid-19 vaccine might differ according to different countries and areas. Hence, it is safer to opt for the vaccines available and continue to wear a mask.

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