Estimating case fatality ratios from infectious disease surveillance data

Knowing which groups are most at risk from an emerging infectious disease is crucial in deciding the optimal allocation of resources during an outbreak response. The case fatality ratio is the fraction of cases who die after contracting a disease. Incomplete reporting of the number of infected individuals, both recovered and dead, can lead to biased estimates of the case fatality ratio. The relative case fatality ratio is the factor by which the case fatality in one group is greater or less than that in a second group. We propose an estimator for the relative case fatality ratio that controls for time-varying reporting rates. We generalize our methods to account for elapsed time between infection and death. We conduct a simulation study to evaluate the performance of our methods in realistic outbreak scenarios. Our new methods could provide valuable information to identify vulnerable subpopulations early in an outbreak of an emerging pathogen such as pandemic H1N1 influenza.