Epidemiologic inference from the distribution of tuberculosis cases in households in Lima, Peru

Abstract

Background: Tuberculosis (TB) often occurs among household contacts of people with active TB. It is unclear whether clustering of cases represents household transmission or shared household risk factors for TB.

Methods: We used cross-sectional data from 764 households in Lima, Peru, to estimate the relative contributions of household and community transmission, the average time between cases, and the immunity afforded by a previous TB infection.

Results: The distribution of cases per household suggests that almost 7 of 10 nonindex household cases were infected in the community rather than in the household. The average interval between household cases was 3.5 years. We observed a saturation effect in the number of cases per household and estimated that protective immunity conferred up to 35% reduction in the risk of disease.

Conclusions: Cross-sectional household data can elucidate the natural history and transmission dynamics of TB. In this high-incidence setting, we found that the majority of cases were attributable to community transmission and that household contacts of case patients derive some immunity from household exposures. Screening of household contacts may be an effective method of detecting new TB cases if carried out over several years.

Figure 1.

Characteristics of cohort study. A, Distribution of ages at first diagnosis. B, Proportion of participants by age (as of 31 December 2005) who had been treated for active tuberculosis (TB); x’s toward right end of horizontal axis denote ages for which there were no data; dashed horizontal line, mean cumulative incidence in persons aged ≥20 years. C, Time lag between diagnoses in the first 2 individuals in the same household with active disease, among households with ≤7 adults; data are binned in 6-month intervals. D, Secondary attack rate, calculated as (average no. of cases in household − 1)/(household size − 1). Sizes of markers are proportional to numbers of households of each size.

Figure 2.

Comparison between data and models for households with 2–7 adults. Point estimates for B and Q were taken as median values from posterior densities. Num., number of; Prop., proportion.

Figure 3.

Posterior probability densities for household and community disease parameters. A, Marginal posterior densities for probabilities of community-acquired disease (solid line), (1 −B), and household-acquired disease (dashed line), (1 − Q). B, Joint posterior density for household and community transmission. Vertical axis indicates probability of observing a particular combination of household (1 −Q) and community (1 − B) transmission. Color of the surface indicates the percentage of nonindex cases in a household that are due to community transmission, rather than household transmission, for each parameter combination; darker colors indicate majority household transmission, and lighter colors, majority community transmission. Both figure parts were produced using 2 million samples for the posterior density.