Influenza surveillance systems monitor the occurrence and progress of the disease so as to support influenza management during epidemics. Clinical and virological influenza surveillance systems have been established in the European member states [1, 2], and the European Centre for Disease Prevention and Control (ECDC) aggregates data regarding influenza occurrence from these systems to enhance monitoring and reporting of disease trends across Europe .
Syndromic surveillance systems based on immediate, usually electronically available, routine health information are increasingly being added to traditional surveillance structures (i.e., clinical / sentinel or virological) to establish more comprehensive surveillance or epidemic intelligence systems [4, 5]. Typically based on the use of existing routine data, the systems do not require new data collection mechanisms. However, since the data are not being collected primarily for surveillance purposes, the provided information covers only signs and symptoms and contains no clinically verified or laboratory-confirmed diagnoses . Due to real-time or near real-time data availability, syndromic surveillance systems are designed to enhance the identification of immediately occurring or out-of-season health threats, such as pandemic influenza. Existing syndromic surveillance approaches apply indicator-based components, such as data from emergency departments [6, 7], emergency medical dispatch centres [8, 9], and telephone help lines [10, 11]; as well as information on school-absenteeism [12, 13] or over-the-counter drug sales of analgesics . The data may be even broader, systems that apply event-based information use information from media sources or web queries related to influenza [15, 16].
European and international syndromic surveillance systems based on event-based health information exist. The Directorate General for Health and Consumers of the European Commission (EC), for example, directs the Medical Information System (MedISys), which monitors the international media for general disease occurrence information but also specifically for influenza activity . Routine syndromic surveillance systems based on indicator-based components, however, are scarce and are, at least in Europe, the individual efforts of single regions or countries. A European study to identify commonalities and good practice in national or regional syndromic surveillance activities has been lacking for a long time and has now been established by an EC co-founded project . The analysis of the potential for a European-wide application of emergency medical service (EMS) data for indicator-based syndromic influenza surveillance is missing so far .
Moreover, existing national and regional EMS data-based syndromic surveillance systems do not focus on the entire chain of available data. Data covering the entire EMS chain consists of out-of-hospital emergency medical dispatch (EMD) information on signs and symptoms typically described by laypeople calling for an ambulance; ambulance service (EP) data on the initial diagnostic findings during examination at the emergency scene by paramedics or emergency physicians; and in-hospital information from nurses or physicians at the emergency department (ED) covering the patient’s main complaints or the initial diagnostic findings during the patient’s treatment in the ED . Typically, however, EMS data-based syndromic influenza surveillance systems focus mostly on ED data, only a few include data from the EMD, and to our knowledge, EP data is not yet exploited by any syndromic influenza surveillance system. Thus, little is known about the differences in the performance of syndromic influenza surveillance based on the three levels of available emergency medical service data and the applicability of this health information for syndromic influenza surveillance in various European countries.
To evaluate the performance of a common syndromic influenza surveillance approach based on the EMD, EP and ED data from different European regions during the autumn/winter wave of the A(H1N1) influenza pandemic, we focus on the validity components, sensitivity and specificity, as well as on timeliness measures as described by Buehler et al. . The validity and timeliness assessment is performed retrospectively against traditional influenza surveillance sources.