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In Chapter 2 we introduced vaccine efficacy parameters that require conditioning on exposure to infection. Household studies were used as the basis for defining exposure to infection in vaccine studies as early as the 1930s in evaluating the efficacy of pertussis vaccines (Kendrick and Eldering 1939). In addition to evaluating vaccine efficacy, household studies have been used to learn about transmission and natural history of many infections. Aspects of the natural history studied in households include the transmissibility, the incubation and latent periods, the duration of infectiousness, and the serial interval between cases (Hope-Simpson 1952; Bailey 1957). Household studies have also been used to evaluate other interventions, such as post-exposure prophylaxis with influenza antiviral agents (Welliver et al 2001; Hayden et al 2004). Exposure to an infectious case within a household can be used as a natural challenge study, for example, when studying immunological correlates of protection (Storsaeter et al 1998). Longitudinal studies of pneumococcal carriage in households and schools have been used to estimate the acquisition and clearance rates for asymptomatic pneumococcal carriage.
The general idea of a transmission unit is that individuals make contact sufficient for transmission within it. Households are the most common form of transmission unit used in studies. It allows easy identification of contacts between a case and susceptibles, and families are convenient units of study. Many other settings are also used as transmission units in studies and analyses that condition on exposure to infection. These include sexual partnerships, classrooms, schools, school buses, airplanes, day care centers, and workplaces, among others. Here we talk mostly about household studies, but many of the study designs and analyses are applicable with possibly slight modification to other transmission units as well. The term household is much easier for exposition than is “transmission unit”.