Abstract
We currently have little idea where Mycobacterium tuberculosis (Mtb) transmission occurs in high incidence settings. Molecular studies suggest that only around 8–19% of transmission to adults occurs within-household, or between known social-contacts. This contrasts with findings from social-contact studies, which show that substantial proportions of contact time occur in households, workplaces and schools. A mathematical model of social-contact behaviour and Mtb transmission was developed, incorporating variation in susceptibility and infectiousness. Three types of contact were simulated: household, repeated (individuals outside household contacted repeatedly with daily-monthly frequency) and non-repeated. The model was parameterised using data from Cape Town, South Africa, on mean and variance in contact numbers and contact durations, by contact type, and fitted to an estimate of overdispersion in numbers of secondary cases (‘superspreading’) in Cape Town. Household, repeated, and non-repeated contacts contributed 36%, 13%, and 51% of contact time, and 13%, 8%, and 79% of disease, respectively. Results suggest contact saturation, exacerbated by long disease durations and superspreading, cause the high proportion of transmission between non-repeated contacts. Household and social-contact tracing is therefore unlikely to reach most tuberculosis cases. A better understanding of transmission locations, and methods to identify superspreaders, are urgently required to improve tuberculosis prevention strategies.
Highlights
We currently have little idea where Mycobacterium tuberculosis (Mtb) transmission occurs in high incidence settings
When we simulated patterns of meetings and transmission, assuming no additional variation in infectiousness or susceptibility, the proportion of tuberculosis cases resulting from transmission between household members in the model was 21.2%, 28.6%, and 37.1% in the high, medium, and low non-repeated casual transmission risk scenarios respectively (Fig. 2), higher than the 8–19% suggested by empirical data
Studies of social contacts suggest that large proportions of indoor meeting time between people occur between household members or close social contacts
Summary
We currently have little idea where Mycobacterium tuberculosis (Mtb) transmission occurs in high incidence settings. In Karonga District, Malawi, only 9.4% of all cases in the community could be attributed to transmission from known contacts - 8.2% family, and 1.2% other known contacts[4] This contrasts with findings from social-contact studies, which show that substantial proportions of contacts and contact time occur in households. We defined meetings where conversation occurred as ‘close contacts’ This included all household and regular, and some non-regular contacts. In one set of scenarios, we simulated no variation between people in infectiousness or susceptibility to tuberculosis (beyond that resulting from extrapulmonary, smear positive, or smear negative disease type, and HIV status). In a second set of scenarios, we simulated realistic levels of individual-level variation in infectiousness and susceptibility, by fitting the model to an empirical estimate of overdispersion in the number of secondary cases generated by each person with pulmonary tuberculosis (‘superspreading’). Using molecular data from tuberculosis cases in Cape Town[13], we estimated k to be 0.15 (95% confidence interval 0.074–0.32)
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