There is much to be gained by responding quickly to the onset of disease spread. In Figure 4.15 we compare the outbreak size when containment is achieved by personal infection control and distancing measures in the SEIRH model when R0 is 1.5. We assume that there are no control measures in place at the start of the outbreak, but that after a number of weeks, transmission is reduced to 70% of its original level outside the household. The left hand plot compares the epidemic curve when personal infection control and distancing is introduced 10 (red), 12 (green) and 15 (blue) weeks after the first case. The right hand plot shows the total outbreak size as a function of the number of weeks delay in implementing the measures. This intervention is sufficient to eliminate disease spread once it is implemented, however delays in implementation can lead to many cases. When the delay is less than 5 weeks, the total outbreak size remains below 2000. When the delay reaches 10 weeks, the total size ranges from 2000 to 100,000 cases. With a delay of 15 weeks, the outbreak reaches close to 1 million cases.

The effect on the total outbreak size is less dramatic if the measures that are introduced are not sufficient to eliminate transmission, though rapid response is still of importance. Figure 4.16 presents the epidemic curves for delays of 0, 5 and 10 weeks in introducing personal infection control and distancing measures when R0 = 2.5.
The outbreak size when personal infection control and distancing measures are introduced into the SEIRH model

    Figure 4.15 The outbreak size when personal infection control and distancing measures are introduced into the SEIRH model with R0 = 1.5 after a delay of a number of weeks. The left hand plot shows the epidemic curves for delays of 10 weeks (red dotted line), 12 weeks (green solid line) and 15 weeks (blue dashed line). The right hand plot shows the total cases as a function of the delay in weeks.

The effect of delay in introducing personal infection control and distancing measures on the epidemic curve and final outbreak size in the SEIRH model
    Figure 4.16 The effect of delay in introducing personal infection control and distancing measures on the epidemic curve and final outbreak size in the SEIRH model with R0 = 2.5. The left hand plot compares epidemic curves when measures to reduce contacts by 30% are introduced immediately (red dotted line), 5 weeks (blue dashed line) and 10 weeks (green solid line) after arrival of infection. The right hand plot shows the final outbreak size as a function of the delay in introducing these measures.

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Using Mathematical Models to Assess Responses to an Outbreak of an Emerged Viral Respiratory Disease(PDF 873 KB)