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Vaccine efficacy is defined as the reduction in the incidence of a disease among people who have received a vaccine compared to the incidence in unvaccinated people. It is usually measured in a randomised controlled trial (RCT) The efficacy of a new vaccine is measured in phase II or phase III clinical trials by giving one group of people a vaccine and comparing the incidence of disease in that group to another group of people who do not receive the vaccine. Ideally the trial is a randomized controlled trial (preferably "double-blind").
- VE = (ARU - ARV)/ARU (x 100)
- VE = vaccine efficacy;
- ARU = attack rate in the unvaccinated population
- ARV = attack rate in the vaccinated population.
Vaccine efficacy differs from vaccine effectiveness in the same way that an explanatory clinical trial differs from an intention to treat trial: vaccine efficacy shows how effective the vaccine could be given ideal circumstances and 100% vaccine uptake; vaccine effectiveness measures how well a vaccine performs when is used in routine circumstances in the community.
- Weinberg GA, Szilagyi PG. Vaccine epidemiology: efficacy, effectiveness, and the translational research roadmap. J Infect Dis 2010;201(11):1607-10 PMID 20402594.
- Orenstein WA, Bernier RH, Dondero TJ, Hinman AR, Marks JS, Bart KJ, et al. Field evaluation of vaccine efficacy. Bull World Health Organ 1985;63(6):1055-68 PMID 3879673.
- Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases (NCIRD). Flu Vaccine Effectiveness: Questions and Answers for Health Professionals. How do vaccine effectiveness studies differ from vaccine efficacy studies? 2011. Last updated: October 12, 2011. Last viewed: December 15, 2011.
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