Modeling the Prevalence and the Force of Infection Directly from Antibody Levels

Abstract

As discussed in previous chapters and illustrated by different methods, prevalence and force of infection (FOI) (and, as we will show in Chaps. 14 and 15, indirectly other parameters such as the basic reproduction number) are estimated from so-called seroprevalence data. Seroprevalence data are obtained by dichotomizing or trichotomizing disease-specific antibody levels using one or two threshold values, often provided by the test manufacturer. In particular, individuals are diagnosed as infected (left-censored age at infection, its value somewhere before the age at the time of the test) if their test result exceeds a certain threshold value τ _{s u} and as being susceptible (right-censored age at infection) if their result falls below a possibly different threshold τ _{s ℓ} ≤τ _{s u} . In case two different threshold values are used (τ _{s ℓ} <τ _{s u} ), individuals having test results in between are labeled inconclusive or equivocal. Individuals labeled inconclusive are either advised to have their sample retested, considered diseased (conservative approach) or non-diseased (liberal approach) or discarded from analysis. Figure 5.1 in Sect. 5.1 showed Belgian Parvovirus B19 antibody activity levels together with two thresholds. This is a situation where both groups can be nicely separated and one can expect a very limited number of misclassified subjects. Figure 11.1 shows a similar plot for data on varicella zoster Virus (VZV, see Chap. 4).