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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?

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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. Jan 18, 2021 at 0:17
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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, 2021 at 4:50
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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. Jan 19, 2021 at 8:35
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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" Jan 19, 2021 at 8:42
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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%" Jan 19, 2021 at 8:47

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