The relative importance of within-household and community transmission of infection among children aged 6 months to 14 years living in a Peruvian Shanty-town, was investigated. The prevalence of mycobacterium TB exposure among 175 contact children (sharing a household with a person who had confirmed pulmonary TB) and 382 control children (living in nearby households free of active TB) was defined as the proportion of children with a positive purified protein derivative (PPD) skin test.

Ninety-seven (55 percent) contact children and 129 (34 percent) controls were PPD positive. Living in a contact household (odds ratio 1. 74, 95 percent CI 1.11-2.73) and age (1, 11, 1.06-1.18) were significant risk factors for PPD positivity. The community infection ratio (CIR) was calculated as the odds ratio of PPD positive controls to PPD-positive contacts:

              Prevalence in controls/ (1-prevalence in controls)
 CIR =   ------------------------------------------------------------------
             (Prevalence in contacts/ (1- prevalence in contacts).

A low CIR therefore suggests mainly household spread of infection, whereas a high value suggests frequent transmission outside the household. The adjusted odds ratio (for age, sex, within -household correlation, and household size) was 0.40 (95 percent CI 0.26-0.64), compared with values of 0.18-0.37 in studies elsewhere. Currently recommended TB control strategies are suitable for areas with low CIR`s. Different strategies may be needed for areas such as the one studied here, with high values.