Influenza

The causative agents of influenza are viruses (orthomyxoviruses), which are subdivided into types A, B and C. Influenza A viruses are named according to type and subtype, e.g. A/H3N2. There are no subtypes for influenza B. Only influenza A and influenza B viruses are relevant for humans.

worldwide

Influenza A viruses occur in humans and also in mammals (pigs, horses). However, the actual reservoir of influenza A viruses are birds, especially waterfowl.

Predominantly through virus-containing droplets when coughing or sneezing

Generally 1-2 days, up to 4 days

Sudden onset of illness with fever (38.5 °C), dry irritating cough and muscle pain and/or headache

Treatment is mainly symptomatic

There is an annual vaccination

As of calendar week (CW) 40/2021, surveillance of the current influenza season through the Clinical Sentinel Surveillance System has begun.

The Austrian Reference Centre for Influenza Epidemiology at the Institute of Medical Microbiology and Hygiene Vienna is responsible for recording the epidemiological situation of influenza in Austria on the basis of a clinical and a virological sentinel surveillance system as well as laboratory reports of influenza virus detections from a further six virological laboratories.

At weekly intervals, the estimated weekly incidence of ILI (influenza like illness) is calculated and published on the AGES homepage Influenza since the beginning of the 2009/2010 season. The data originate from the sentinel ILI surveillance system established since 1992/1993, which consists of the influenza information system of Magistrate 15 of the City of Vienna, the influenza information system of Dept. 7 of the City of Graz and the influenza surveillance system Greater Innsbruck. Since the beginning of 2012, ILI case data from all three influenza information systems Vienna, Graz and Innsbruck are processed once a week by the Reference Centre for Influenza Epidemiology and sent to the Federal Ministry of Social Affairs, Health, Care and Consumer Protection (BMSGPK) for transmission to TESSy (The European Surveillance System) and from TESSy to WHO/EuroFlu.

Clinical surveillance

In 1992/93, the ILI (influenza-like illness) sentinel system was established: 52 registered volunteer reporting physicians (general practitioners and paediatricians) from the influenza information systems of Magistrate 15 of the City of Vienna and Dept. 7 of the City of Graz as well as the influenza surveillance system of the Innsbruck area report weekly the cases of ILI identified within one working week according to the definition of influenza-like illness (ILI) to the Reference Centre for Influenza Epidemiology. Here, the estimated incidence per calendar week is calculated at weekly intervals (number of reported cases per number of inhabitants of the patient catchment area of the reporting physician).

Virological surveillance

Virological surveillance is performed by the virological sentinel system DINÖ (Diagnostic Influenza Network Austria), coordinated by the National Reference Laboratory for Influenza Viruses at the Centre for Virology of the Medical University of Vienna. 98 sentinel physicians (= reporting physicians) send nasopharyngeal swabs of ILI cases to the National Reference Laboratory for Influenza every week for testing. The weekly number of specimens tested for influenza and the number of specimens with influenza virus detection are sent by the National Reference Laboratory to the Reference Centre for Influenza Epidemiology.

Another five influenza diagnosing laboratories (Section of Virology, Dep. of Hygiene, Microbiology, Social Medicine; Med. University IBK, Tyrol, Department of Virology; Infectious Serology, Institute of Hygiene, Microbiology, and Environmental Medicine; Medical University Graz, Microbiological Laboratory; Joint Practice for Travel Medicine, IBK, Tyrol, Analyse BioLab GmbH; Companies of Elisabethinen Linz and AGES, Upper Austria, SALK Labor GmbH, Salzburg) also report once a week the weekly number of samples with influenza virus detection according to virus type or subtype as well as the number of samples tested for influenza. Subtyping of circulating influenza viruses performed by the National Reference Laboratory is used to detect the emergence of new influenza virus variants and to compare them with the strains contained in the current vaccine. The aim of the Austrian Influenza Surveillance System is to monitor influenza activity in order to detect seasonal as well as inter-seasonal influenza epidemics at an early stage.

Mortality

As influenza is often not recognized or registered as a cause of death, it is international standard that influenza-related deaths are estimated by modelling. Such a model was established in Austria in cooperation of the National Reference Centre for Influenza Epidemiology of the AGES (PD Dr. Daniela Schmid; DI Lukas Richter) with the National Reference Laboratory for Influenza at the Department of Virology of the Medical University of Vienna (Univ.-Prof. Dr. Theresia Popow-Kraupp; Dr. Monika Redlberger-Fritz).

Final analysis for the 2019/2020 season (as of 10.06.2020, evaluation until week 20/2020):

Table 1: Estimated number of deaths associated with seasonal influenza (IA) and temperature extremes (ET) including 95% confidence interval (CI) for the seasons 2015/2016-2019/2020 (each week 40-KW 20 of the following year), Austria.