Vaccine Field Studies
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This chapter is an introductory chapter to vaccine field studies, i.e. vaccine efficacy and vaccine effectiveness studies. The difference between vaccine efficacy and vaccine effectiveness is explained, and the two major infection occurrence measures, attack rate and force of infection, are introduced. The formula that links the two measures, the so-called exponential formula, is explored in detail. The impact of the diagnostic test on vaccine efficacy and vaccine effectiveness estimates is inspected. The chapter is concluded with an investigation of the use of specific and non-specific endpoints in vaccine field studies. A formula is presented for the relationship between vaccine effectiveness against such endpoints and vaccine effectiveness against infection. It is shown that the general belief that a non-specific endpoint as surrogate for infection will always lead to underestimation of the vaccine effectiveness is false.