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Considering the fact that vaccine efficacy may be a difficult concept for physicians and health officials alike, we decided to explain it using data from the first publication on the efficacy and safety of a COVID-19 vaccine produced by Pfizer/BioNTech. We examined the published data and calculated common epidemiological parameters such as RRR (relative risk reduction), RR (relative risk), ARR (absolute risk reduction) and NNT (number needed to treat) for 3 groups of patients as described in the original paper. Further, we calculated safety parameters for the vaccine as NNH (number needed to harm) for any, related and severe side effects as mentioned by the investigators. We argue that both NNT and NNH are necessary estimates of how a vaccine might perform in real life and that a robust understanding of efficacy is vital for patients and health care providers as well as health officials in order to make responsible and balanced policy decisions regarding vaccination.
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