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I've been performing many web searches using combinations of the words "vaccine", "vaccination", "COVID-19", "coronavirus", "efficacy", and "effectiveness", and have yet to find anything that provides an accurate answer to the following question:

When a vaccine manufacturer states that their vaccine is "95% effective", does that mean:

A. 100% of people receiving the vaccine during studies developed antibodies from the vaccine, but 5% still got sick. (Which, I think, would actually mean it is less than "95% effective" because not everyone would get exposed to the virus that causes the illness.)

(or)

B. 95% (or less) of the study participants receiving the vaccine developed antibodies from the vaccine, therefore 5% of the people who received it still got sick.

Which is it (or is it something else), and if it is "A", is my parenthetical understanding correct?

Although my interest lies predominantly regarding COVID-19 vaccines, I think this question is general enough that it could apply to any vaccination.

End Anti-Semitic Hate
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  • It means roughly "the likelihood that you get the virus is reduced by 95% relative to a control group that did not get the vaccine". – BrenBarn May 01 '21 at 05:49
  • @BrenBarn Right, but why? Reason A or B? That is the question posed above. – End Anti-Semitic Hate May 01 '21 at 06:58
  • It is about whether people contract the disease, not about antibodies. See the duplicate question for more info. But it is neither your A or B, because in both cases your description makes no reference to a control group. The 95% number says nothing about 95% of people in the trial group. It says the likelihood of getting the disease in the trial group is 95% lower than in the control group. The proportion of people who got COVID in the vaccine trial was far lower than 5%. – BrenBarn May 01 '21 at 21:00
  • @BrenBarn Thanks. That makes sense and I think it reflects what I have read elsewhere (I'm very tired, so my brain needs rest right now). I'm going to close my question and then after I have more time to think about it, write a question that more accurately asks what I'm trying to learn. Thank you again for your help. – End Anti-Semitic Hate May 01 '21 at 23:30

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