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]
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
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…
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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