title: Direct and indirect effects in vaccine efficacy and effectiveness.
abstract: In 1915, Greenwood and Yule noted that for valid vaccine efficacy studies, exposure to infection in the vaccinated and the unvaccinated must be equal (Proc R Soc Med 1915;8(part 2):113-94). The direct effect of a vaccine, however, needs to be defined by the protection it confers given a specific amount of exposure to infection, not just a comparable exposure. In this paper, two classes of parameters are distinguished along lines differing from the conventional distinction between efficacy and effectiveness. Efficacy parameters attempt to control for exposure to infection and represent direct effects on individuals. Direct effectiveness parameters represent a mixture of direct effects on individuals and indirect effects in the population.
authors.affiliation: Division of Biostatistics, Emory University School of Public Health, Atlanta, GA 30329.
authors.forename: M E
authors.initials: ME
authors.lastname: Halloran
authors.forename: M
authors.initials: M
authors.lastname: Haber
authors.forename: I M
authors.initials: IM
authors.lastname: Longini
authors.forename: C J
authors.initials: CJ
authors.lastname: Struchiner
mesh_terms: D004781:Environmental Exposure; D006801:Humans; D015994:Incidence; D017053:Infection Control; D007239:Infections / Q000453:epidemiology; D011157:Population Dynamics; D011897:Random Allocation; D014611:Vaccination / Q000706:statistics & numerical data