How Not To Read Vaccine Efficacy

To address COVID-19 pandemic, many vaccines have received emergency approval (that is, by bypassing many regulatory requirements that are otherwise needed for vaccines or other therapeutic interventions). The third phase trial data for relative risk reduction (RRR) are referred to as efficacy of the vaccines. 

A popular perception is that if an individual takes the vaccine then the possibility of being afflicted by the disease gets reduced by this RRR%. This, however, is not the case. Then, what does this efficacy mean? From a lay perspective, RRR% is total risk in per cent (100%) minus relative risk in per cent (RR%).

Now, what is relative risk (RR) in a trial. It is the ratio of the share of those afflicted by the disease in the treatment arm of the trial (SDT=nt/Nt, number afflicted by the disease in the treatment arm of the trial divided by number of people in the treatment arm of the trial) to the share of those afflicted by the diseased in the control arm of the trial (SDC=nc/Nc, number afflicted by the disease in the control arm of the trial divided by number of people in the control arm of the trial). In other words, RR=SDT/SDC and when it is multiplied with 100 it gives us RR%.

The absolute risk reduction in per cent (ARR%=SDC%-SDT%) is the proportion of those afflicted by the disease in the control arm of the trial (SDC%) minus the proportion of those afflicted by the disease in the treatment arm of the trial (SDT%).   

A recent Lancet Microbe paper reports that for five COVID-19 vaccines that have received emergency approval the ARR% are as follows: 

1.28% Oxford-AstraZeneca (ChAdOx1 nCoV-19), 

1.24% Moderna-NIH (mRNA 1273), 

1.19% Johnson & Johnson (Ad26.COV2.S), 

0.93% Gamaleya (GamCovidVac) [Sputnik V], and 

0.84% Pfizer-BioNTech (BNT162b2 mRNA).  

However, the RRR% for these are:

66.84% Oxford-AstraZeneca (ChAdOx1 nCoV-19), 

94.08% Moderna-NIH (mRNA 1273), 

66.62% Johnson & Johnson (Ad26.COV2.S), 

90.97% Gamaleya (GamCovidVac) [Sputnik V], and 

95.02% Pfizer-BioNTech (BNT162b2 mRNA).

Note that the reporting of RRR% are at times in misleading ways, as a Lancet Infectious Diseases paper shows. An informed consent, among other things, should also provide this perspective to the vaccine recipients before they take a decision.