Page 98 - Timeliness of Infectious Disease Notification & Response Systems - Corien Swaan

96 Chapter 4
equal to the medians. For each disease we evaluateAd the ratio at which PIR1 and PIR2 are reduced after the reporting delay median is reduced by 1 day, as extracted from the OSIRIS database. These reduction ratios allowed us to determine those diseases for which reduction of reporting delays would most prevention transmission.
Underreporting
Our calculations of PIR1 and PIR2 were made with the assumption of 100% reporting compliance. However, a proportion of cases are not reported (and might include asymptomatic cases). From an outbreak control point of view, underreporting can be tackled by assuming that there are only reported cases, each producing an increased average number of infections to account for the
contribution to disease transmission from the cases which are not reported (Fi- gure 3, panel C). PIR1 and PIR2 are modified as follows: PIR (underreported) = PIR x (1 - proportion.underreported) + proportion.underreported. Therefore, there is a maximum limit for underreporting beyond which it is not possible to satisfy the outbreak control condition R x PIR(underreported) <1. We calculated this limit for the studied diseases by assuming instantaneous reporting at the day of symptom onset. Although this assumption is not realistic, it provides an upper limit estimate for underreporting if outbreak control is desired.
Vaccination Coverage
Many diseases are preventable by vaccination; among the 13 diseases in the National Immunization Program vaccination schedule for the Netherlands are
hepatitis B, measles, mumps and pertussis. Consequently, part of the popula- tion might be protected by vaccine-induced immunity. When considering that individually targeted interventions are implemented effectively, the minimum vaccination coverage needed for achieving herd immunity is reduced. We cal- culated this reduction for the 6 studied diseases by first considering the current reporting delays and the outbreak control condition [R(1 - coverage)]2 x PIR2<1. From this condition we derived the reduced minimum vaccination coverage needed for outbreak control and compared it with the standard vaccination co- verage needed for achieving herd immunity (coverage>1 - 1/R).
Model Parameters
We fitted log-normal distributions to frequency distribution of reporting delays as extracted from OSIRIS. We performed fitting with a Kolmogorov-Smirnov mi- nimization and a 0.05 significance level using the program Mathematica version
B
C