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I've had my first Pfizer vaccine and the second is scheduled 11 weeks after the first one. I may have to postpone it though. If I cancel it, what is the latest date I can take the second dose without invalidating (?) the first one? Can I postpone it another month?

I know clinical trials have answered these question.

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  • Why 11 weeks? That's already well outside the guidelines. – Carey Gregory Jun 14 at 15:21
  • In the UK 11 weeks is the soonest you're allowed to get it. – Pete Jun 14 at 15:24
  • Canada is doing 12 weeks, down from 16 (in April, when people were booking their first and second appointments in a single process) – Kate Gregory Jun 15 at 14:48
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I know clinical trials have answered these question

They have not. The reason for the specific windows specified for vaccination is that these windows are what was studied in the trials. Other windows were not studied. Therefore, it is unknown what level of protection will be provided outside that window. It could even be better, but is probably about the same.

It's reasonable to infer that small deviations from the instructions will not matter much, and that a second dose of the vaccine any time will be better than never getting it, but that's all. Similar questions have been asked and answered by experts in all sorts of lay press, here's one example: https://www.scientificamerican.com/article/is-it-safe-to-delay-a-second-covid-vaccine-dose/

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