Page 97 - Timeliness of Infectious Disease Notification & Response Systems - Corien Swaan

Quantifying reporting timeliness to improve outbreak control 95
intervention. B) PIR1 and PIR2 values when index cases are notified and stopped to- gether with their secondary cases, according to a time distribution. C) How PIR values in panel B are modified by 40% underreporting. Dark grey shading indicates PIR1 and PRI2 values. The black line indicates the proportion of index cases not yet notified (right y-axis), equivalent to the probability of an index not yet being notified in each situation. PIR 1, expected proportion of cases caused by index case at notification; PIR 2, expected proportion of new infections caused by secondary cases before index case is notified.
Calculation of PIR2, a 2-Generation-based Response
The calculation of the proportion of expected infections produced by secondary cases at the time when their corresponding index case (PIR2) is conducted in the same way as that of PIR1, but a 2-generation interval is used instead of a standard generation interval. A 2-generation interval is constructed by subse- quently adding up 2 standard generation intervals. The 2-generation interval distribution indicates the average second-generation infective profile as time passes since the index acquired the pathogen (Figure 3, panel A). PIR2 is then provided by the area under the 2-generation interval curve weighted by the probability of the index not yet being reported (Figure 3, panel B).
At an early stage, an outbreak is controlled (i.e., prevalence begins to decli- ne) by implementing a case-based intervention that can stop transmission early enough so that the number of cases produced per infector is < 1 (6,8,9). The number of cases produced per index case is calculated by multiplying PIR1 times the reproduction number (R) of the disease in question. The number of cases produced per secondary case is PIR2 multiplied by the reproduction number. Hence, for each disease, we assumed that the conditions for outbreak control are PIR1<1/R and PIR2<1/R. In addition, given that PIR2 involves 2 generati- ons, we considered the alternative (more restrictive) outbreak control conditi- on PIR2<1/R2. To evaluate the status of the current reporting timeliness of the 6 diseases, we compared our PIR1 and PIR2 results with the outbreak control conditions.
To assess the potential for improvement by reducing current reporting de- lays for each disease, we studied how much various reporting delays influence PIR1 and PIR2, and calculated the reporting delay reductions needed to reach the outbreak control conditions. PIR1 and PIR2 were highly dependent on the reporting delay median but not so regarding standard deviations within the range matching actual reporting delay distributions (online Technical Appendix Figure). Therefore, for simplicity, we present our reporting delay analysis results as medians of 1-60 days and the assumption that the standard deviations are
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