In my blog How Not To Read Vaccine Efficacy based on a Lancet Microbe paper it was shown that five COVID-19 vaccines with emergency approval (that is, by bypassing certain regulatory requirement) had relative risk reduction (RRR=1-RR; RR=SDT/SDC, relative risk is the ratio of the shares exposed to disease in treatment and control arms) that ranged from 67% to 95% but, their absolute risk reduction (ARR=SDC-SDT) ranged from 0.84% to 1.28%.
The Pfizer-BioNTech (BNT162b2 mRNA) as per its phase 3 trial has an RRR of 95%. This immediately gives the impression that the proportions of the shares exposed to disease are 1% for treatment and 20% for control, but this could be 2% and 40% or any in other proportions that satisfy k times 1% and k times 20%.
If the proportions for SDT and SDC are 1% and 20%, respectively, then ARR will be 19%. But, ARR is actually 0.84%, which suggests that in the trial k=0.044. And, hence, SDT=0.04% and SDC=0.88%.
It is difficult to replicate this in a population, as design and methods cannot be the same as in a trial. However, following mass vaccination in Israel using Pfizer-BioNTech (BNT162b2 mRNA), a study shows that RRR is 94% (almost close to what was observed in the trial), but ARR was 0.046% (SDT=0.02% and SDC=0.48%) suggesting that k=0.024 is much lower than the trial.
A further decline in ARR means that the number of people that need to be vaccinated (NNV=100/ARR) to prevent one more person from being exposed to the disease is further increased by 83% from 119 in the trial to 217 in the specific population setting. In other words, the vaccine effect was further reduced in a population setting when compared to the trial.
The authorities associated with mass vaccination need to design studies to arrive at RRR and ARR, along with other aspects, in their population, as that would be of help in their public policy designing. The individual concerned should also have such information, along with other aspects, for them to take an informed decision.
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