Not counting the intended double dose of the 2-dose vaccines such as Moderna and Pfizer.

I can imagine this in several instances. Say someone threw away their vaccine card after the second shot, and later learn they need that card to document their vaccination for an employer or school, and getting vaccinated again is the most expedient way to solve that. Or, they aim to do international travel, but they don't think their vaccine is on the list of that country's accepted vaccines, so they get a vaccine that is.

Are there known risks to doubling up on vaccines generally?

Is anything documented about the various COVID vaccines that warns against taking them when already vaccinated?

There is an existing question but it was confused for a "what if you split doses 1 Pfizer 1 Moderna" question.

  • Could you please state the benefit you are looking for? This could potentially be dangerous. Moderna/Pfizer has 95% coverage puts this at 5% change in values which isn't statistically significant.
    – William
    May 23, 2021 at 3:41
  • @William, one example of a benefit: the Johnson & Johnson vaccine is known to be at least 60% effective against the B.1.351 variant, while the AstraZeneca vaccine is at best 10%, and the effectiveness of Moderna and Pfizer are unknown. Meanwhile, Moderna and Pfizer are 95% effective against baseline variants, versus Johnson & Johnson's 70%.
    – Mark
    May 23, 2021 at 5:02
  • 1
    @William, that graphic, like most intended for popular consumption, doesn't take variants into account. AstraZeneca's uselessness against B.1.351 is well-known, and is the reason why South Africa returned their supply unused: that's the dominant variant there.
    – Mark
    May 24, 2021 at 2:39
  • My inferred answer is probably, yes. In clinical trials, the placebo and non-placebo groups have to be told whether they got the placebo or non-placebo partially to avoid vaccinating somebody twice. This also allows those that didn't get the placebo to get the actual vaccine.
    – William
    Jun 3, 2021 at 1:50


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