What does Vaccine Efficacy actually mean?

Public Health Scotland & University of Edinburgh study showed the Pfizer/BioNTech vaccine reduced hospitalisations by 85% and 94% for the Oxford AstraZeneca vaccine. It is important to recognise that this is still early data but it is still highly encouraging.

What does Vaccine Efficacy actually mean?

The headline figures for Vaccine Efficcacy released over the last few months four each vaccine are based on ability of a vaccine to prevent any symptomatic disease (so preventing COVID symptoms of any kind).

If a Vaccine for example had a 82% efficacy, it doesn’t mean that 82% of people that get vaccinated won’t get symptomatic COVID and 18% will. It instead means that there in the vaccine trial there were 82% fewer cases of COVID in the vaccinated group than there were in the unvaccinated group.

This study however did not look at Efficacy of the vaccine in preventing symptomatic, instead it looked at Vaccines Efficacy in preventing hospitalisations.

We are all well aware that people in older age groups are more likely to have suffer from a severe COVID infection that requires hospitalisation. So in a relatively short period of time after starting to vaccinate that age group you’ll quickly start to see the true effect of these vaccines.

We continue to see a high number of admissions in this age group. We start to see however that most of these admissions involve people who had yet to be vaccinated, with only a small proportion made up of people who have already been vaccinated.

If hypothetically there were 106 hospital admissions amongst the over 65’s taking part in the study; 100 of these admissions being individuals who are not vaccinated and 6 being vaccinated individuals, Efficacy in preventing hospitalisation would be 94%. In this example I am working on the basis that there were the same number of vaccinated/unvaccinated people in the study groups. We can however still easily determine Vaccine Efficacy if the number of people in the study are not split equally between the vaccinated and unvaccinated groups. You simply need to take into account the number of people in each group when determining the Attack rate for each group (Vaccinated/unvaccinated).

Why does the efficacy of the vaccines appear lower amongst under 65’s earlier in this study?

We see that Vaccine Efficacy at 14-20 days in this study was less than 40% for Under 65’s and over 60% for over 65’s. This seems the opposite of what you would expect? There is however a logical explanation behind this.

In the Under 65’s group in the study, there will be far fewer people who suffer severe disease requiring hospitalisation. Therefore early on in the study there will only have been a very small number of admissions amongst the Under 65 group in the study.

For example at this early stage we may have only seen 5 admissions, with 3 individuals being unvaccinated and 2 admission being a vaccinated individual. This would result in a calculation of Vaccine efficacy of 40%. You can’t however accurately determine the Efficacy of the vaccine for the Under-65’s based on such a small number of admissions.

A few weeks on, there had been more people hospitalised in this group. Let us say that hypothetically we saw 100 admissions amongst the under 65’s in the study. If 85 of these admissions are individuals who had not been vaccinated and 15 individuals who had, Vaccine Efficacy will increase to 85%.

Why does the Oxford AstraZeneca vaccine appear to have a higher efficacy than the Pfizer vaccine?

I can think of a couple of reasons that could potentially explain why the Oxford AstraZeneca vaccine had a  higher efficacy in this study. 

The first being that Scotland 🏴󠁧󠁢󠁳󠁣󠁴󠁿 prioritised vaccinations in Care homes and so utilised the initial Pfizer stocks on those older people most vulnerable to COVID infection. The Oxford AstraZeneca vaccination likely seeing more use amongst independent individuals in better health living in the community.

The second is that Vaccines utilising virus vectors such as the Oxford AstraZeneca vaccine generate a higher proportion of their overall effectiveness through the 1st dose. The T cell levels being close to the maximum just 2-3 weeks after the 1st dose of the vaccine. 

The differences between the Oxford AstraZeneca vaccines are not very significant when it comes to T cell levels that are thought to be a key component of protecting against severe disease.

The Pfizer vaccine does produce higher levels of neutralising antibody after two doses, which is the likely reason why after 2 doses it has a higher efficacy in preventing symptomatic disease. The levels of neutralising antibody being the crucial factor in preventing infection.If the antibodies can prevent infection, they will also prevent disease (both symptomatic and asymptomatic).

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