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cnbc.com (mirror) mentions:

The FDA says Pfizer’s Covid vaccine is safe and effective. But trial participants warn of intense symptoms after second shot.

Why does the Pfizer’s Covid vaccine cause more intense intense symptoms after second shot then after the first shot?

Franck Dernoncourt
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    I think most/all two-dose vaccines do this. Simply because you already have some specific anti-bodies etc. on the 2nd dose. – the gods from engineering Jan 18 '21 at 00:17
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    Agree with @Fizz. I found the HBV vaccine memorable. First dose was nothing, second dose left my arm a bit sore, and third dose felt like someone whacked me in the shoulder with a 2x4. I always interpreted that as a reflection of the building immunity. – Carey Gregory Jan 18 '21 at 04:50
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    @CareyGregory: I guess to disprove me for ChAdOx1 "The overall reactogenicity profile after the second dose was lower than after the first". OTOH for Moderna's vaccine the same pattern as for Pfizer's can be seen in the data. – the gods from engineering Jan 19 '21 at 08:35
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    The ChAdOx1 paper even emphasized this diff: "The observation of reduced second-dose reactogenicity is in contrast to reported profiles of two mRNA vaccines for COVID-19 and a protein-adjuvant vaccine technology, in which, although generally well tolerated, reactogenicity increased with the second dose" – the gods from engineering Jan 19 '21 at 08:42
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    @CareyGregory: I suspect however that that's because (or at least correlated with the fact that) a higher second dose would probably be more effective for ChAdOx1 given that in the accidental administration of half-strength 1st dose ultimate immunity was better for that vaccine. "It has been widely reported that the Oxford COVID-19 had an overall efficacy of just 70% compared to 90% for the Pfizer and Moderna mRNA vaccines. But those people who were given half the original dose for the first injection had an efficacy of 90%" – the gods from engineering Jan 19 '21 at 08:47
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    "The efficacy in those people receiving the correct dose according to protocol was only about 62% compared to over 90% found in the previously reported mRNA vaccine results." (last quote from https://www.sciencemediacentre.org/expert-reaction-to-phase-3-interim-analysis-of-the-oxford-astrazeneca-covid-19-vaccine/) – the gods from engineering Jan 19 '21 at 08:47
  • Relevant: https://medicalsciences.stackexchange.com/q/25759 – Stilez Jun 04 '21 at 04:50

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