Vaccine efficacy: how well does a vaccine work?

What does "efficacy" actually mean and how is it calculated from trial data?
16 February 2021

Interview with 

Stephen Evans, London School of Health and Hygiene




In the present game numbers are everything, and here to unpick them for us is Stephen Evans, a medical statistician at the London School of Hygiene and Tropical Medicine, speaking with Chris Smith...

Stephen - In my area of about 5,000 people, over the next week there could easily be a hundred people who end up with Covid-19. If they're unvaccinated, then those hundred people will get the disease. If they're vaccinated with a vaccine with a 90% efficacy, then 90 of them will be protected from getting any symptoms of the disease. There will be 10 who will still get symptoms. Some of those 10 may be protected from more severe disease, but that's what the 90% means. It doesn't mean that each individual is 90% protected, but about 90% of the people who would otherwise get it are protected and 10 are less protected.

Chris - The values we're looking at for AstraZeneca's vaccine are somewhere hovering around 80%, aren't they? And the values returned for Pfizer's vaccine, after two doses, are said to be in the region of 95%. So given what you've just told us, if I have a vaccine that's for the sake of argument, let's say 95% effective, and that means that five and every hundred people won't be protected and could catch it. Does that mean then that a person getting the AstraZeneca vaccine is in theory worse off than someone getting the Pfizer vaccine because they've got a higher likelihood that they might not be protected?

Stephen - Yes, there is a slightly higher risk of getting mild disease. Whether there is any extra risk of being hospitalised and getting severe disease, we don't fully know, but it's quite likely that the AstraZeneca vaccine will still protect - even those who get mild disease are unlikely to be hospitalised or die.

Chris - What do you think is going on with the Europeans then? Because our MHRA gave the AstraZeneca vaccine the nod. The equivalent across Europe, the EMA, also approved its use. But then some countries said that they wouldn't be approving it for older people because they didn't have enough data, they're saying. And this has obviously led to a number of people being quite concerned who are in the older age bracket and were destined to receive or have received AstraZeneca's vaccine. What should they take away from this information?

Stephen - You need to distinguish approved and prioritised. If you have plenty of the AstraZeneca vaccine and plenty of the Pfizer vaccine, then you might need to decide, "who will we prioritise?" And so it is reasonable to say, we will prioritize the Pfizer vaccine to the elderly if we have plenty of it, because we have better evidence in the elderly. Now it doesn't mean that the AstraZeneca doesn't protect them. It's just that the trials didn't have enough people who caught the disease to be able to demonstrate that you didn't get mild disease. But the data on the antibodies that we just heard about, and the data on the T cells and B cells, suggest that the elderly will be protected just as much. So that's why the EMA and the MHRA have said these are authorised in older people. So there is no need to be concerned that they don't have efficacy, but you need to prioritise depending on the context you're in. And we have lots of people in care homes who would not find it easy to get the Pfizer vaccine because of its cold chain. And so our decision to give them the AstraZeneca vaccine is eminently sensible.

Chris - And South Africa have stirred up some controversy because they're sitting on million scale numbers of doses of AstraZeneca's vaccine and have called a halt to its rollout citing, as far as we know, one small trial from the University of Witwatersrand where the average age in that trial was 31. And there were no people in it who got severe disease, but they said they didn't feel that it was adequate data in terms of the protection confirmed by AstraZeneca's vaccine to use it. Where do you think they're coming from?

Stephen - I think that's very strange. I think that if you're then going to say that I'm happy to have a whole lot of people who die unprotected by any vaccine, that's a very strange decision to make. Okay. It's entirely possible the AstraZeneca vaccine will be less effective, but making comparisons between vaccines is very difficult when you haven't done the actual comparison.


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