However, during research, it is possible that an intervention actually increases the risk of participants, for example, in the STEP and Phambili studies, which were both intended to test an experimental HIV vaccine .In these cases, the formula would yield a negative efficacy value because ARV>ARU. A negative efficacy value is sometimes present in the lower limit of a confidence interval of an estimate of vaccine efficacy for specific clinical endpoints. While this means that the intervention may actually have a negative effect, it could also be simply due to small sample size or sample variability.
https://en.wikipedia.org/wiki/Vaccine_efficacy
https://en.wikipedia.org/wiki/Vaccine_efficacy
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