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. 2007 Aug 1;166(3):355-63.
doi: 10.1093/aje/kwm082. Epub 2007 May 10.

Estimating variability in the transmission of severe acute respiratory syndrome to household contacts in Hong Kong, China

Virginia E Pitzer  1 , Gabriel M Leung, Marc Lipsitch
Affiliations

Estimating variability in the transmission of severe acute respiratory syndrome to household contacts in Hong Kong, China

Virginia E Pitzer et al. Am J Epidemiol. .

Abstract

The extensive data collection and contact tracing that occurred during the 2003 outbreak of severe acute respiratory syndrome (SARS) in Hong Kong, China, allowed the authors to examine how the probability of transmission varied from the date of symptom onset to the date of hospitalization for household contacts of SARS patients. Using a discrete-time likelihood model, the authors estimated the transmission probability per contact for each day following the onset of symptoms. The results suggested that there may be two peaks in the probability of SARS transmission, the first occurring around day 2 after symptom onset and the second occurring approximately 10 days after symptom onset. Index patients who were aged 60 years or older or whose lactate dehydrogenase level was elevated upon admission to the hospital (indicating higher viral loads) were more likely to transmit SARS to their contacts. There was little variation in the daily transmission probabilities before versus after the introduction of public health interventions on or around March 26, 2003. This study suggests that the probability of transmission of SARS is dependent upon characteristics of the index patients and does not simply reflect temporal variability in the viral load of SARS cases.

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Figures

FIGURE 1.
FIGURE 1.
Probability of transmission of severe acute respiratory syndrome (SARS) according to day of index case illness during the 2003 SARS epidemic in Hong Kong, China. The solid line represents the estimated mean transmission probability, while the dotted, dash-dotted, and dashed lines represent the 80%, 90%, and 95% credible intervals (CIs), respectively.
FIGURE 2.
FIGURE 2.
Estimated gamma distribution for the incubation period (time from infection to symptom onset) of severe acute respiratory syndrome (SARS) during the 2003 SARS epidemic in Hong Kong, China.
FIGURE 3.
FIGURE 3.
Probability of transmission of severe acute respiratory syndrome (SARS) according to day of index case illness during the 2003 SARS epidemic in Hong Kong, China, stratified by date of symptom onset (before vs. on or after March 26). The solid lines represent the estimated mean transmission probabilities, while the dotted, dash-dotted, and dashed lines represent the 80%, 90%, and 95% credible intervals, respectively.
FIGURE 4.
FIGURE 4.
Probability of transmission of severe acute respiratory syndrome (SARS) according to day of index case illness during the 2003 SARS epidemic in Hong Kong, China, stratified by the lactate dehydrogenase (LDH) level of the index case (high, low, or not measured). The solid lines represent the estimated mean transmission probabilities, while the dotted, dash-dotted, and dashed lines represent the 80%, 90%, and 95% credible intervals, respectively.
FIGURE 5.
FIGURE 5.
Probability of transmission of severe acute respiratory syndrome (SARS) according to day of index case illness during the 2003 SARS epidemic in Hong Kong, China, stratified by age of the index case (≥60 years vs. <60 years). The solid lines represent the estimated mean transmission probabilities, while the dotted, dash-dotted, and dashed lines represent the 80%, 90%, and 95% credible intervals, respectively.
FIGURE 6.
FIGURE 6.
Sensitivity of statistical analyses to heterogeneous infectiousness in simulated data for a hypothetical population. Plots show the estimated transmission probabilities for (a) unconfounded heterogeneity, (b) confounded heterogeneity, and confounded heterogeneity when data are stratified on (c) normally infectious index cases and (d) highly infectious index cases. The solid lines represent the estimated mean transmission probabilities, while the dotted, dash-dotted, and dashed lines represent the 80%, 90%, and 95% credible intervals, respectively. The thick black line represents the “true” population mean probability.
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