Resolution:
## Figure 1.
Deriving multiplication factors from the morbidity surveillance pyramid. A: The morbidity surveillance pyramid is often used to illustrate the availability
of morbidity data at each surveillance level. With each ascending level (from the
community, to healthcare institutions (GPs, hospital, laboratory), to regional and
national public health agencies); data availability shrinks and only a fraction of
cases from the level below is captured
[7-9]. In contrast to the narrow tip of the pyramid which represents data held by national
public health agencies, the base is wide as it holds all infections in the community.
The difference between the number at the tip and base can be considered cases lost
to 'underestimationâ€™ (UE). B: The proportions of infections that are symptomatic, that attend healthcare, and
that are reported are represented in this decision tree model. Here, only 55% of all
infected individuals attending healthcare are reported through the notification system.
If 1000 cases were reported then a MF of 1.8 (=100/55) could be derived and would
correct for those underreported cases. The true number attending healthcare would
be 1800 cases. Likewise, if only 60% of symptomatic cases attended healthcare, then
a MF of 1.7 (=100/60) would correct for under-ascertainment of symptomatic cases.
The true number of cases attending healthcare would be 3000 symptomatic cases (=1.7*1800).
Finally, since 90% of infections were symptomatic, a MF of 1.1 (=100/90) would correct
for under-ascertainment of asymptomatic cases. The true number of infections would
be 3300 (=1.1*3000). A MF to correct for total underestimation of symptomatic cases
in one step would be 3.06 (=1.8*1.7) and for all infections 3.4 (=1.8*1.7*1.1). 'All
infectionsâ€™ shaded in orange in Figure
1A represents the same population as the orange box in Figure
1B. 'Cases reportedâ€™ in blue in Figure
1A represents the same population as the blue box in Figure
1B.
Gibbons |