1/ The MHRA has approved a longer gap between doses for both the AstraZeneca vaccine and the Pfizer vaccine. The latter has concerned some people. Specifically many are citing a figure of 52% for protection after the first dose.
Here is why this 52% figure isn't useful [1/n]
Here is why this 52% figure isn't useful [1/n]
2/ The 52% value is a real figure, which comes from the Pfizer trial, for the period between the first and second doses. Here is what that period of time looked like (fda.gov/media/144325/d…):
In red are people who received placebo and in blue are those who received the vaccine
In red are people who received placebo and in blue are those who received the vaccine
3/ We can see that until day 3 we have near identical results in both groups. This is *expected* - no vaccine has an effect until days later when the immune system has had time to develop a response against it.
4/ As an immune response builds the lines slowly diverge from each other. By day 10 they have completely different trajectories. We do have to be careful here, analysing the data post-hoc. But it is clear that (as one expects a priori) full protection isn't realised immediately.
5/ *The 52% figure is the average protection over these 21 days*, so it includes that initial time before the immune system has had time to create a response.
6/ If one instead looks at the day 10 to day 22 period one instead gets an efficacy value of 86% (there will be confidence intervals around that).
Analysis here: theosanderson.github.io/adhoc_covid/pf…
Analysis here: theosanderson.github.io/adhoc_covid/pf…
7/ Whereas if one looks at the day 0 to day 10 period there is an efficacy of 10%.
8/ The 52% figure is a mush of those two completely different scenarios. It's not useful. People shouldn't be citing it in this context.
9/ The Moderna vaccine documents (similar vaccine class) actually break down the results after the first dose into 14 day periods and show a very similar effect fda.gov/media/144434/d…
10/ There's lots to reasonably discuss about single-dosing. The big question is what the efficacy against severe COVID is in days 21-90. But please stop using this 52% figure.
11/ Document from the JCVI featuring essentially the same analysis: app.box.com/s/uwwn2dv4o2d0…
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