India approved the Moderna jab this week, the fourth Covid-19 vaccine that will be used in the country. Moderna has confirmed its shot has an efficacy of 94 per cent, higher than Covishield, Covaxin, and Sputnik V.
But if you think it means 94 out of 100 people who got the US-made jab are protected from Covid-19 infections, you are wrong.
So what does vaccine efficacy really mean then in general? Let us understand this with Moderna's example.
Efficacy Rate
Moderna's 94 per cent efficacy means if 100 people are infected with Covid-19, only six on average would have got ill if they were all vaccinated. So, if a trial result shows a 94 per cent efficacy, it means a vaccinated person is 94 per cent less likely to be infected than a person without a vaccine.
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In a nutshell, the efficacy value indicates a relative reduction in infection risk and not an absolute probability of protection against Covid-19 despite being vaccinated.
The Trial Math
Drugmakers run a trial among selected participants of different layers of the population divided into two groups: half get the vaccine, and half a placebo (without any active substance or a dummy vaccine).
After a certain period (nine weeks after the second dose in the case of the Moderna), researchers count how many people got infected from Covid-19 in these two groups.
In the Moderna trial on 28,207 of the 196 participants, for instance, eleven people from the vaccinated group and 185 from the placebo group were found infected.
Relative efficacy is determined by dividing the number of infected people of the vaccinated group by the placebo group and subtracted from 1.
Here's the simple math:
1-(11/185) x 100 =94.1%
"The primary endpoint of the Phase 3 COVE study was based on the analysis of COVID-19 cases confirmed and adjudicated starting two weeks following the second dose of vaccine. This final analysis was based on 196 cases, of which 185 cases of COVID-19 were observed in the placebo group versus 11 cases observed in the Moderna COVID-19 Vaccine group, corresponding to a 94.1% vaccine efficacy," Moderna said in its December statement.
Experts said the important insight is that it's not the overall number of participants in the study that is relevant here but the number of people who got infected with Covid-19.
"The vaccine efficacy determined in the trials are just a snapshot of the virus in the world at that given time. When the vaccine goes out into the general population, the prevalence of the virus might have changed and any variants that may be circulating. These factors may affect the real-world effectiveness of the vaccine. However, in the case of the Moderna vaccine clinical trial data and what we are now seeing in people of different age groups, the vaccine efficacy is holding, so its suffice to say that vaccine is effectiveness is very similar as seen in clinical trials," Dr Priya Luthra, principal investigator in viral diseases research and translational science program at Trudeau Institute in New York, said to India Today.
The Efficacy Caveat
The efficacy of all vaccines is mathematically derived with the same method. The difference in trial scenarios and total exposure of participants may give different rates for different vaccines.
That said, efficacy based on clinical trials is not necessarily translated into the efficacy of the jabs in the real world.
Covid-19: What does vaccine efficacy really mean? - India Today
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