Vaccine efficacy: beyond the average

Tuan Nguyen
6 min readJan 15, 2021

Australian experts are debating about the efficacy of AstraZeneca vaccine. Some argue that the efficacy (VE) of the vaccine is not good enough for herd immunity; others say that it is better than … face mask. I think the debate has been misled by the heavy reliance on the average. In this note, I want to make 3 comments on the true value of VE and its implication for herd immunity.

Australia is about to implement a COVID-19 vaccine program, with Oxford/AstraZeneca vaccine being the intervention of choice. It is expected that up to 80,000 people will be vaccinated by week 4, and 4 million by the end of March 2021. It is going to be a major public health effort.

However, some experts (eg immunologists and infectious disease specialists) are calling for a rethink of the program. They quoted the vaccine efficacy of 62% and argued that such a degree of efficacy is not high enough to achieve herd immunity [1].

Other experts, including the Government’s Chief Medical Office (CMO), argue that the vaccine efficacy is good enough. An expert quoted the efficacy of ‘up to 70%’ and said that was a great news. He reminds us that the efficacy of mask wearing was only 15–20% [1]. The CMO went on to advise that we should listen to the experts, because “they are hand picked experts on these matters”.

Which vaccine efficacy?

Well, in science we have the saying, “In God we trust, all else bring data”. Surely, experts’ opinions are important, but scientifically their opinions are ranked lowest in evidence-based medicine. As can be seen, in terms of vaccine efficacy, some experts quoted the 62% figure, others quoted the 70% figure. Who is correct? Which number is correct?

We need to see the original data. The data are available from the Lancet [3], and the key data are reproduced in the table below. Also, please pay attention to the title of the paper, ‘an interim analysis’, suggesting that the reported data are preliminary. That fact should be kept in mind when we interpret the data.

Estimates of vaccine efficacy (from the Lancet paper)

However, before interpreting the data, we should to pay attention to a glitch in the study. According to the study’s protocol, all participants were supposed to receive two standard doses (SD) of the vaccine. However, due to a serendipity of an error…



Tuan Nguyen

osteoporosis | epidemiology | genetics | biostatistics | data enthusiast