Page 99 - Timeliness of Infectious Disease Notification & Response Systems - Corien Swaan
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Quantifying reporting timeliness to improve outbreak control 97
8.0 (http:www.wolfram.com/mathematica/). Table 1 shows the parameters we found and used in our models for each disease.
Because generation intervals are difficult to observe, we used the serial interval as a proxy for the generation interval (10) and assumed equivalence. The serial interval is the time between symptom onset of an index case and symptom onset of a secondary case. For each disease we extracted information on incubation period distribution, serial interval and reproduction number from the published literature (Table 1). Most data on incubation periods and serial intervals in the literature is provided as a range with a relevant value (average, median or mode). Therefore, we constructed the distributions by finding pa- rameters for which a log-normal distribution would present the relevant value found in the literature, and its 2.5th and 97.5th percentiles would correspond to the ranges found in the literature. Log-normal distributions are easy to handle and there is evidence favouring them as incubation period distributions (11).
Results
Current Reporting Timeliness
Current reporting timeliness is shown in Figure 1; calculations are based on data from the Netherlands and consider interventions applied only to the reported index case. For most diseases, the expected proportion of infections produced until reporting is >90%. The expected proportion of infections is lower for he- patitis only; however, this proportion is still high at >80%. Therefore, if an index case is instantly removed as a source at the moment of reporting, in general ≤10% secondary infections are prevented, which renders such an intervention rather ineffective. Even fewer infections can be prevented if underreporting is considered. All diseases shown in Figure 4 lie above outbreak control conditions.
When interventions are also applied on secondary cases produced by a re- ported index case it becomes more effective. The expected proportion of infec- tions produced by secondary cases, PIR2, for the 6 diseases is shown in Figure 5. Hepatitis A lies close to the lower outbreak control condition, indicating that its current reporting speed should be timely enough to keep it under control. This is indication applies for hepatitis B which also lies below the upper out- break control limit, despite its intermediate PIR2 values. PIR2 for measles is in- termediate, which places the disease far outside the area where control is pos- sible. PIR2 is low to intermediate for mumps, but the reproduction number for mumps is smaller than that for measles, which places the disease close to the
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