Appropriate measures of influenza immunization program effectiveness

Abstract

Groll and Thomson's evaluation of the effectiveness of Ontario's Universal Influenza Immunization Campaign used per capita cases of laboratory-confirmed influenza. We argue that these data are susceptible to various biases and should not be used as an outcome measure. Laboratory data are traditionally used to identify the presence of influenza activity rather than to identify levels of influenza activity. A better measure of viral activity is the proportion of influenza tests positive; whereas the weekly proportion of tests positive was relatively consistent, a marked increase over time in the numbers of laboratory-confirmed cases paralleled an increase in the number of tests performed. Regardless, for evaluating universal influenza immunization program effectiveness, other established and available measures employed in previous studies describing the epidemiology of influenza should be used instead of laboratory data.

Fig. 1

Comparison of weekly number of laboratory-confirmed cases of influenza A and B (a) with number of tests for influenza performed (b) for Ontario, 1993–2004. Introduction of Ontario's Universal Influenza Immunization Program (UIIP) in October 2000 is indicated by the solid vertical line and the outbreak of severe acute respiratory syndrome (SARS) in the spring of 2003 is indicated by the dashed vertical line.

Fig. 2

Comparison of weekly number of laboratory-confirmed cases of influenza A and B (a) with proportion of tests positive (b) for Ontario, 1993–2004. UIIP introduction of is indicated by the solid vertical line and the SARS outbreak is indicated by the dashed vertical line.