Page 104 - Timeliness of Infectious Disease Notification & Response Systems - Corien Swaan

102 Chapter 4
Discussion
Public health agencies are responsible for organising a swift course of action from disease onset to notification and intervention. The general assumption is that the shorter the delay, the better the control response. We quantitatively assessed the potential of individual-based interventions by PHAs for minimizing or preventing outbreaks by calculating the expected proportion of infections caused by an index (PIR1) and secondary (PIR2) cases until reporting of the in- dex. For hepatitis A, measles, mumps, pertussis and shigellosis, PIR1 was >90% (Figure 3), but for hepatitis B, PIR1 was 82%. This finding indicates that interven- tions aimed only at notified index cases are always too late to substantially pre- vent further disease transmission. Therefore, effective control requires contact tracing and stopping transmission from contacts. Even if reporting delays would be reduced to a couple of days, interventions targeting index cases only are not enough to achieve outbreak control conditions.
The expected proportion of infections caused by secondary cases, PIR2, dif- fers substantially among different diseases. Current reporting delays for hepatitis A and hepatitis B lead to PIR2 values that are within outbreak control limits (Fi- gure 5), probably because of the long incubation time of each disease. Although incubation periods for pertussis and shigellosis differ greatly, reporting for these diseases is far from meeting outbreak control conditions (Figure 5), probably resulting partly from patient and physician reporting delays because of nonspe- cific symptoms. Measles and mumps appear in the middle of Figure 5, despite their relatively short incubation periods, probably because they produce specific symptoms. For measles and mumps, a combined effort to lower their reproduc- tion number and shorten notification delay might bring their reporting within outbreak control conditions. Lower reproduction number numbers move disea- ses to a lower position in Figures 4 and 5. This lowering can be achieved with in- terventions at population or group level (e.g. vaccination or hygiene/behaviour changes). Areas with high baseline vaccination ratios against measles or mumps (boosters included) are closer to meeting outbreak control conditions than tho- se with low baseline vaccination ratios and need less drastic delay reductions to achieve the same effect when interventions are applied to secondary cases.
Whether a PHA response to a reported case is considered timely depends on the goal set for the intervention and the balance between the benefit and efforts spent on reducing reporting delays. PIR2 decreases significantly with a reporting delay reduction of a few days for hepatitis A, measles and mumps. This finding suggests that efforts to reduce reporting delays for these disea-