Early presentation of an infected traveler increases the probability that this traveler’s chain of transmission becomes extinct, although even immediate presentation at onset of symptoms fails to ensure that an epidemic is not initiated, as is shown in Figure 3.6.

The effects of R0 and the time (days) from the onset of symptoms to presentation at a medical centre on the probability that an undetected infected arrival initiates an epidemic.

    Figure 3.6 The effects of R0 and the time (days) from the onset of symptoms to presentation at a medical centre on the probability that an undetected infected arrival initiates an epidemic.

Nevertheless, decreasing the time from the appearance of symptoms to seeking medical help, with subsequent isolation, has a relatively small effect on the delay distribution. For example, compared with typical presentation and subsequent isolation, having infected travelers present 6 hours following the onset of symptoms increases the median delay from 56 to 60 days, when 400 travelers arrive from the source region per day. Reducing the delay to presentation to 0 (clearly for illustrative purposes only) only increases the delay a further 2 days (Figure 3.7).
Combining perfect symptomatic border screening, presentation at 6 hours following symptom onset with a drastic reduction in traveler numbers (10 per day) produces no substantial synergies. Assuming R0=1.5, the median delay increases from 56 to 85 days, with most of this delay attributable to the reduction in traveler numbers alone, as shown in Figure 3.1.

The effect of the time from symptoms to presentation (tSP) on the distribution of the time delay until an epidemic reaches 20 concurrent cases in Australia following identification in the source country (assumed to occur when there are 10 concurrent cases
    Figure 3.7 The effect of the time from symptoms to presentation (tSP) on the distribution of the time delay until an epidemic reaches 20 concurrent cases in Australia following identification in the source country (assumed to occur when there are 10 concurrent cases in infected areas). Calculations assume R0=1.5, a peaked infectiousness function, 400 travelers/day attempting to depart the source country, and no other interventions.

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