Two models estimating the proportion of Escherichia coli O157[ratio ]H7 cases not reported in the
Ontario notifiable diseases surveillance system are described. The first model is a linear series
of adjustments in which the total number of reported cases is corrected by successive under-reporting coefficients. The structure of the second model is based on a relative difference in the
proportion of E. coli O157[ratio ]H7 cases which are hospitalized between the surveillance database
and the underlying population.
Based on this analysis, the rate of under-reporting of symptomatic cases of E. coli O157[ratio ]H7
infection in Ontario ranges from 78 to 88% corresponding to a ratio of 1 reported case for
approximately 4–8 symptomatic cases missed by the surveillance system. This study highlights
the need to increase awareness among public health workers of the potential biases that may
exist in the interpretation of routine surveillance data.