calculating efficacy
- related: Biostats
- tags: #literature #pulmonary
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The 2-month vaccine efficacy seen with this novel COVID-19 vaccine in this hypothetical randomized controlled trial is 93%. Vaccine efficacy measures the proportionate reduction of COVID-19 disease among vaccinated subjects. "Vaccine efficacy" is the term used when the vaccine is tested under ideal conditions, as described in this randomized controlled trial. In contrast, "vaccine effectiveness" is a term used when a vaccine is tested under real-world circumstances. Vaccine efficacy/effectiveness (VE) is measured by calculating the risk of disease among vaccinated and unvaccinated persons and determining the percentage reduction in risk of disease among vaccinated persons relative to unvaccinated persons as represented in the following formulas:
VE = (Risk of disease in unvaccinated subjects − Risk of disease in vaccinated subjects)/Risk of disease in unvaccinated subjects
In this formula, the numerator (risk of disease among unvaccinated subjects − risk of disease among vaccinated subjects) is sometimes called the "risk difference" or "excess risk":
VE = 1 − Risk ratio = Risk of disease in vaccinated subjects / Risk of disease in unvaccinated subjects
The calculation of VE using the numbers from the table (Figure 1) is represented as follows:
COVID-19 disease measured 2 months after vaccination was diagnosed in eight of 10,000 vaccinated subjects compared with 110 of 10,000 unvaccinated subjects (placebo).
- Risk of disease among vaccinated subjects = 8/10,000 = 0.0008 = 0.08%
- Risk of disease among unvaccinated subjects = 110/10,000 = 0.011 = 1.1%
- Risk ratio = 0.0008 / 0.011 = 0.07
- VE = (0.011 − 0.0008) / 0.011 = 0.0102 / 0.011 = 0.93 = 93%
- Alternatively, VE = 1 − RR = 1 − 0.07 = 0.93 = 93%
VE is interpreted as the proportionate reduction in disease among a vaccinated group. A VE of 93% indicates a 93% reduction in COVID-19 disease occurrence among vaccinated subjects, or a 93% reduction from the number of cases expected if not vaccinated. In addition, the risk ratio below 1.0 indicates a decreased risk or protective effect for the exposed (vaccinated) subjects. The risk ratio of 0.07 indicates that vaccinated subjects were only 7% likely to develop COVID-19 as were unvaccinated subjects. The VE measured as in this example 2 months after vaccination should not be used to predict VE at other time points beyond the 2-month estimation as described in this randomized controlled trial.1