Harmonisation of national influenza surveillance morbidity data from EISS: a simple index

The European Influenza Surveillance Scheme is a collaboration with 18 member countries (2001/02) which monitors the activity and impact of influenza by collecting morbidity and virological data in primary care facilities throughout the winter season each year. Despite being in principle similar in the surveillance concept, the indicators used and observations made are very different. Different healthcare systems and organisational needs (eg a certificate of illness for the employer) influence the consultation behaviour. Furthermore, and partly as a result of differences in the healthcare systems, the definitions used for the numerator and denominator when calculating morbidity rates are different. Thus comparative interpretation of participating countries' morbidity data is extremely difficult. Reporting 'harmonisation' by using equivalent numerators and denominators is one option but is difficult to achieve in the short term. Moreover, several additional issues would need to be considered, for example, the need for continuity of surveillance and whether such steps would indeed result in direct comparability etc. A simple index was tested, through which the impact of influenza morbidity in any one year is compared with what is considered a 'usual' epidemic in that country. The index in principle describes numerically the extent to which the influenza-attributable excess morbidity in the current epidemic in each country is within, exceeds, or is less than a range typical for an influenza epidemic. In this pilot study, the usefulness of such an index is explored with the example of eight countries for the seasons 1999/2000 and 2000/01. A fine tuning of the methods has not yet been performed.