The Absolute Risk Reduction is the actual reduction in risk of something happening in a population. So if you have two groups and 20% of the untreated group get the disease but only 12% of the treated group get the disease the ARR is 8%. In laymans’ terms you could say that you have an 8 in 100 chance of getting a benefit from the treatment. An excellent and fuller explanation is here in a free book chapter.
It’s really the measure that should be used for vaccines and other drug treatments but is very dependent on your prior risk of something. If your risk of the same thing was 2% (e.g. because you’re younger) and it dropped by the same proportion on treatment to 1.2%, the ARR would be 0.8% - similar to that touted as the result in the Pfizer study.
Because the level of reduction or impact of a drug might be consistent across different groups, irrespective of their background risk, and because it sounds more impressive, drug companies will often use the Relative Risk Reduction (RRR) - which is just the % drop in the incidence rate.
ARR vs RRR, calculated from official data.
(((They))) knew.