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. 2005 Sep 1;162(5):479-86.
doi: 10.1093/aje/kwi230. Epub 2005 Aug 2.

Methods for estimating the case fatality ratio for a novel, emerging infectious disease

A C Ghani  1 , C A Donnelly, D R Cox, J T Griffin, C Fraser, T H Lam, L M Ho, W S Chan, R M Anderson, A J Hedley, G M Leung
Affiliations

Methods for estimating the case fatality ratio for a novel, emerging infectious disease

A C Ghani et al. Am J Epidemiol. .

Abstract

During the course of an epidemic of a potentially fatal disease, it is important that the case fatality ratio be well estimated. The authors propose a novel method for doing so based on the Kaplan-Meier survival procedure, jointly considering two outcomes (death and recovery), and evaluate its performance by using data from the 2003 epidemic of severe acute respiratory syndrome in Hong Kong, People's Republic of China. They compare this estimate obtained at various points in the epidemic with the case fatality ratio eventually observed; with two commonly quoted, naïve estimates derived from cumulative incidence and mortality statistics at single time points; and with estimates in which a parametric mixture model is used. They demonstrate the importance of patient characteristics regarding outcome by analyzing subgroups defined by age at admission to the hospital.

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Figures

FIGURE 1.
FIGURE 1.
Illustration of the adapted Kaplan-Meier method for an epidemic showing 1 minus the estimated survivorship function for the time from hospital admission to death (bottom line) and the estimated survivorship function for the time from hospital admission to recovery (top line). When the epidemic is complete, these two curves meet. Before this time, they do not meet, and the case fatality ratio is estimated on the basis of the past pattern of deaths/recovery (dotted line).
FIGURE 2.
FIGURE 2.
a) Incidence of cases of severe acute respiratory syndrome (SARS) in Hong Kong in 2003 by date of hospital admission; b) time from admission to the hospital to death/ discharge for the Hong Kong SARS cases.
FIGURE 3.
FIGURE 3.
a) Estimates of the case fatality ratio and 95 percent confidence intervals over the course of the epidemic of severe acute respiratory syndrome in Hong Kong in 2003. Black square, case fatality ratio eventually observed for patients admitted to the hospital by the time denoted on the x-axis; black diamond, first simple estimate (deaths/cases); black triangle, second simple estimate (deaths/(deaths + recovered)); white square, adapted Kaplan-Meier (KM) method; black dotted line, range obtained from the adapted KM method; white circle, gamma mixture model. b) KM curves for the nonparametric survival (bottom lines) and discharge probabilities (top lines) obtained from the data sets on April 9 (short dashed line), April 16 (continuous line), and April 23 (long dashed line).
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