How Effective are the CV-19 Vaccines?

This Comment piece published in the Lancet explores the dangers between the efficacy advertised by the vaccine manufacturers (Relative Risk Reduction – RRR) compared to the Absolute Risk Reduction (ARR).

Manufacturer Advertised Efficacy (RRR)

Moderna – 94%

Pfizer – 95%

AstraZenica – 67%


Absolute Risk Reduction

Moderna – 1.2%

Pfizer – 0.84%

AstraZenica – 1.3%


 “Vaccine efficacy is generally reported as a relative risk reduction (RRR). It uses the relative risk (RR)—ie, the ratio of attack rates with and without a vaccine—which is expressed as 1–RR. Ranking by reported efficacy gives relative risk reductions of 95% for the Pfizer–BioNTech, 94% for the Moderna–NIH, 90% for the Gamaleya, 67% for the J&J, and 67% for the AstraZeneca–Oxford vaccines. 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.

ARR is also used to derive an estimate of vaccine effectiveness, which is the number needed to vaccinate (NNV) to prevent one more case of COVID-19 as 1/ARR.

NNVs bring a different perspective: 76 for the Moderna– NIH, 78 for the AstraZeneca–Oxford, 80 for the Gamaleya, 84 for the J&J, and 117 for the Pfizer–BioNTech vaccines. The explanation lies in the combination of vaccine efficacy and different background risks of COVID-19 across studies: 0·9% for the Pfizer–BioNTech, 1% for the Gamaleya, 1·4% for the Moderna–NIH, 1·8% for the J&J, and 1·9% for the AstraZeneca–Oxford vaccines.

ARR (and NNV) are sensitive to background risk— the higher the risk, the higher the effectiveness—as exemplified by the analyses of the J&J’s vaccine on centrally confirmed cases compared with all cases: 8 both the numerator and denominator change, RRR does not change (66–67%), but the one-third increase in attack rates in the unvaccinated group (from 1·8% to 2·4%) translates in a one-fourth decrease in NNV (from 84 to 64).”




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