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I assume the choice of 2 doses was based on antibody response. From https://www.theguardian.com/world/2021/jan/19/single-covid-vaccine-dose-in-israel-less-effective-than-we-hoped the second dose achieves a 6 to 12 times increase in antibodies. I guess that is because the immune system is already primed to create antibodies. The question that I raised in another comment, is what happens if the doses are spaced 2 months or more apart. Does anyone know?

I agree that any result based on a weeks data would have so few events that it would produce useless results.


Ken: I'm not liking the switch to looking at antibody response. For one, if that is the right measure, then all of the trials should be based on that outcome but none of them do. Secondly, how many vaccinated people did they test for antibody response? I can't imagine this is a large number. And I agree with you that a priori, all three major vaccine developers tested two doses in Phase 3. In fact, AstraZeneca changed their testing protocol mid-way through the trial to go from one to two doses!


Kaiser: I agree that antibody response is a poor surrogate especially as they don't seem to have any data linking vaccine efficacy to antibody response. Presumably at some stage Astra Zeneca realised that their vaccine in a single dose form wasn't good enough. It then didn't help that they had a manufacturing problem and initially produced half strength doses. This seems to have been a massive oversight, as AstaZeneca should have been testing the manufacturing contractors product.

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