* The quoted 95% etc. numbers are in terms of Relative Risk (ie, the ratio of attack rates with and without a vaccine—which is expressed as 1–RR)
* These numbers are essentially a reported efficacy and useless to compare between studies held under different conditions.
With this in mind, from the lancet article linked above:
> However, RRR should be seen against the background risk of being infected and becoming ill with COVID-19, which varies between populations and over time. Although the RRR considers only participants who could benefit from the vaccine, the absolute risk reduction (ARR), which is the difference between attack rates with and without a vaccine, considers the whole population.
>__ARRs tend to be ignored because they give a much less impressive effect size than RRRs: 1.3% for the AstraZeneca–Oxford, 1.2% for the Moderna–NIH, 1.2% for the J&J, 0.93% for the Gamaleya, and 0.84% for the Pfizer–BioNTech vaccines.__
“ ARRs tend to be ignored because they give a much less impressive effect size than RRR”
This is an odd statement. Why would that be the reason to ignore it? ARR is extremely specific to a time frame and location.
Without wide deployment of vaccines, almost everyone would eventually naturally catch the coronavirus at some point in the next several years. As vaccines are deployed, the risk reduces since vaccines reduce the R0 of the virus in that population. Any calculation of ARR would be highly speculative and have to include models about how widely the vaccine would be deployed, the transmissibility and mutation of the virus, and the RRR of the vaccine itself, as well as being stated only for a specific timeframe and location.
Given all this it’s obvious why RRR is the typically reported number and the one that makes sense to use in discussions.
* The quoted 95% etc. numbers are in terms of Relative Risk (ie, the ratio of attack rates with and without a vaccine—which is expressed as 1–RR) * These numbers are essentially a reported efficacy and useless to compare between studies held under different conditions.
With this in mind, from the lancet article linked above:
> However, RRR should be seen against the background risk of being infected and becoming ill with COVID-19, which varies between populations and over time. Although the RRR considers only participants who could benefit from the vaccine, the absolute risk reduction (ARR), which is the difference between attack rates with and without a vaccine, considers the whole population.
>__ARRs tend to be ignored because they give a much less impressive effect size than RRRs: 1.3% for the AstraZeneca–Oxford, 1.2% for the Moderna–NIH, 1.2% for the J&J, 0.93% for the Gamaleya, and 0.84% for the Pfizer–BioNTech vaccines.__