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. 2010 Aug 15;51(4):401-8.
doi: 10.1086/655129.

Tuberculosis Transmission to Young Children in a South African Community: Modeling Household and Community Infection Risks

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Free PMC article

Tuberculosis Transmission to Young Children in a South African Community: Modeling Household and Community Infection Risks

Robin Wood et al. Clin Infect Dis. .
Free PMC article

Abstract

Background: Tuberculosis transmission is determined by contact between infectious and susceptible individuals. A recent study reported a 4% annual risk of child tuberculosis infection in a southern African township. A model was used to explore the interactions between prevalence of adult tuberculosis infection, adult-to-child contacts, and household ventilation, which could result in such a high annual risk of tuberculosis infection.

Methods: Number of residents per household and tuberculosis incidence were derived from a household census and community tuberculosis registers. Using the Wells-Riley equation and probability analyses of contact between infectious adults with tuberculosis and preschool children, we estimated the annual risk of tuberculosis infection within and outside of the home.

Results: There was a mean of 2.2 adults per child-containing household with a 1.35% annual adult smear-positive tuberculosis notification rate. The maximal household annual risk of tuberculosis infection was 3%, which was primarily determined by the number of resident adults. Transmission risk outside the home increased with increasing number of households visited. Transmission probabilities were sensitive to exposure time, ventilation, and period of adult infectivity. The benefits of increased ventilation were greatest when the period of infectivity was reduced. Similar reductions in household transmission could be achieved by increasing ventilation from 2 to 6 air changes/hour or by separating child and adult sleeping areas.

Conclusions: The annual risk of tuberculosis infection of preschool children predominantly results from infectious residents in the home. However, even with limited social interactions, a substantial proportion of transmissions may occur from nonresident adults. The benefits of increased ventilation are maximized when the period of infectivity is reduced by prompt treatment of source cases.

Conflict of interest statement

Conflict of Interest Statement: No conflicts of interest to declare

Figures

Figure 1
Figure 1
The proportions of households with pre-school children in which different numbers of adults are resident. 2083 adults and 1051 pre-school children were resident in 918 households. Data derived from a 2008 household survey performed in Site M.
Figure 2A&B
Figure 2A&B
A: The effect of ventilation (air changes per hour, ACH) and mean period of infectivity (delta, Δ) on the mean annual risk of TB infection resulting from a child sleeping in a shack shared with adults. Values are plotted for mean periods of adult infectivity of 30, 60, 90 and 120 days. B: The effect of period of infectivity (delta, Δ) and ventilation (air changes per hour, ACH) on the mean annual risk of TB infection resulting from a child sleeping in a shack shared with adults. Values are plotted for 2, 6 and 12 air exchanges per hour (ACH). N.B. The period of infectivity (delta, Δ) is the mean time from onset of infective tuberculosis until initiation of effective antituberculosis chemotherapy. Modeled estimations are based on a potential night-time exposure of 12 hours, a median of 2.2 adult residents per shack, a 1.35% annual risk for smear-positive tuberculosis and a mean production of 1 infectious air-borne quantum of TB per hour during untreated smear-positive disease.
Figure 2A&B
Figure 2A&B
A: The effect of ventilation (air changes per hour, ACH) and mean period of infectivity (delta, Δ) on the mean annual risk of TB infection resulting from a child sleeping in a shack shared with adults. Values are plotted for mean periods of adult infectivity of 30, 60, 90 and 120 days. B: The effect of period of infectivity (delta, Δ) and ventilation (air changes per hour, ACH) on the mean annual risk of TB infection resulting from a child sleeping in a shack shared with adults. Values are plotted for 2, 6 and 12 air exchanges per hour (ACH). N.B. The period of infectivity (delta, Δ) is the mean time from onset of infective tuberculosis until initiation of effective antituberculosis chemotherapy. Modeled estimations are based on a potential night-time exposure of 12 hours, a median of 2.2 adult residents per shack, a 1.35% annual risk for smear-positive tuberculosis and a mean production of 1 infectious air-borne quantum of TB per hour during untreated smear-positive disease.
Figure 3A&B
Figure 3A&B
The mean annual risk of TB infection for a child visiting 1 to 3 households other than own residential household during the day each with (Figure A) ventilation of 2 air changes per hour and (Figure B) ventilation of 6 air changes per hour. Values are plotted for mean periods of adult TB infectivity (delta, Δ) of 30, 60, 90 and 120 days. N.B. The period of infectivity (delta, Δ) is the time from onset of infective tuberculosis until initiation of effective antituberculosis chemotherapy. Modeled estimations are for a pre-school child spending 75% of day-time indoors, a median of 2.2 resident adults per visited shack, a 1.35% annual risk for smear-positive tuberculosis and a mean production of 1 infectious air-borne quantum of TB per hour during untreated smear-positive disease.
Figure 3A&B
Figure 3A&B
The mean annual risk of TB infection for a child visiting 1 to 3 households other than own residential household during the day each with (Figure A) ventilation of 2 air changes per hour and (Figure B) ventilation of 6 air changes per hour. Values are plotted for mean periods of adult TB infectivity (delta, Δ) of 30, 60, 90 and 120 days. N.B. The period of infectivity (delta, Δ) is the time from onset of infective tuberculosis until initiation of effective antituberculosis chemotherapy. Modeled estimations are for a pre-school child spending 75% of day-time indoors, a median of 2.2 resident adults per visited shack, a 1.35% annual risk for smear-positive tuberculosis and a mean production of 1 infectious air-borne quantum of TB per hour during untreated smear-positive disease.

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