Tuberculosis transmission to young children in a South African community: modeling household and community infection risks.
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.
Wood
Robin
R
Desmond Tutu HIV Centre, Institute of Infectious Diseases and Molecular Medicine, University of Cape Town, Faculty of Health Sciences, Cape Town, South Africa. robin.wood@hiv-research.org.za
Johnstone-Robertson
Simon
S
Uys
Pieter
P
Hargrove
John
J
Middelkoop
Keren
K
Lawn
Stephen D
SD
Bekker
Linda-Gail
LG
D000293:Adolescent; D000328:Adult; D002648:Child; D002675:Child, Preschool; D017714:Community-Acquired Infections / Q000453:epidemiology* / Q000635:transmission*; D005192:Family Health; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D015233:Models, Statistical; D015995:Prevalence; D018570:Risk Assessment; D013019:South Africa / Q000453:epidemiology; D014376:Tuberculosis / Q000453:epidemiology* / Q000635:transmission*