In 8,895 participants who received two full doses, the efficacy of the AstraZeneca vaccine was 62%. But due to a dosing error that turned out to be a happy accident, among the 2,741 volunteers who received a half dose followed by a full dose, the efficacy of the vaccine rose to 90%.
That means that, overall, when both dosage forms are considered, the vaccine showed a 70% efficacy rate.
Why efficacy is not the same as efficacy
Given that the Oxford University-AstraZeneca candidate vaccine also claims up to 90% efficacy (Pfizer / BioNTech claims 95% efficacy, Moderna 94.5%, and Sputnik-V 92%), it might seem that efficacy is the same as efficacy, but the two have very different meanings.
While efficacy is essentially a measurement tool during clinical trials, effectiveness is the performance of the vaccine in the real world and, generally, effectiveness is always less than efficacy, if the performance of past vaccines is a indication.
During clinical trials, volunteers are divided into two groups, some are given the vaccine while others are given a placebo. Based on the number of people who get sick in each group, researchers determine effectiveness.
For example, of the 43,661 volunteers in the Pfizer-BioNTech vaccine candidate, 170 people tested positive for Covid-19 and showed their symptoms. Of these, 162 were those who had received the placebo injection, while the remaining eight received the vaccine. Based on this, the researchers calculated the proportion of volunteers in each group who became ill, and the proportion of unvaccinated volunteers was much higher than the proportion of vaccinated volunteers.
The difference between the two is what is known as efficacy; for example, if none of those who got sick had been vaccinated, the efficacy is 100%, while if there is no difference between the vaccinated and placebo groups, the efficacy is zero.
The reason why efficacy tends to be lower than efficacy is because in clinical trials, the volunteers recruited are often healthy and not representative of demographics.
In reality, large sectors of the population can suffer from some chronic health problems, such as diabetes, which in fact has been shown to be often fatal for those who contracted Covid-19. These chronic health problems could potentially limit the effectiveness of a vaccine in protecting people against the pandemic. Also, efficacy also doesn’t tell you what your chances are of getting sick after getting vaccinated.
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