The causative agents of influenza are viruses (orthomyxoviruses), which are subdivided into types A, B, C and D. Relevant for humans are influenza A and influenza B viruses, which trigger seasonal epidemics every year.



Pathogen reservoir

Influenza A virus can infect both humans and animals (including pigs and ferrets, wild birds, and poultry). Influenza B virus generally infects humans; in isolated cases, seals have also been affected. Influenza C virus is rarely detected, generally results in rather mild infections, and therefore has no public health significance. Influenza D virus mainly infects cattle; human cases are not known.

Infection route

Predominantly through virus-containing droplets when coughing or sneezing

Incubation time

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


For patients with uncomplicated influenza infection, treatment focuses on symptom relief. Patients with severe or progressive clinical disease with suspected or confirmed influenza virus infection should be treated with antiviral medications as soon as possible.


The most important and effective preventive measure is vaccination. Influenza viruses evolve rapidly, so vaccination is adjusted every year.

Annual vaccination is generally recommended, with priority given to people over the age of 60, people with certain chronic diseases, groups of people with other risk factors, and healthcare and elderly care workers.

Situation in Austria

In Austria, the annual flu season continues throughout the winter months. Flu outbreaks of varying intensity occur regularly. Several systems are used in Austria to monitor the flu situation, and monitoring of the current flu season by the Clinical Sentinel Surveillance System began in calendar week (CW) 40/2023.

The SARI dashboard shows inpatient admissions to Austrian hospitals with diagnoses of severe acute respiratory infections (SARI). These include influenza (flu), COVID-19, RSV and other severe respiratory diseases.

Estimated number of influenza/flu-like illnesses/100,000 inhabitants per calendar week (CW), Austria, CW 40-14, 2023/2024

Estimated number of influenza/illness-like illnesses (ILI)/100,000 population:in per age group and per calendar week, Austria, calendar weeks 40-14, 2023/2024

Estimated number of influenza/illness-like illnesses (ILI)/100,000 population, reported ARI-related sick leave/100,000 eligible insured/working population as of CW, CW 40-14, 2023/2024.

Number of laboratory-confirmed cases of influenza A, influenza B, and estimated number of influenza/flu-like illnesses/100,000 population per week, Austria, weeks 40-14, 2023/2024

Number of laboratory-confirmed cases by influenza virus type/subtype and percent of influenza samples testing positive among sentinel samples tested per CW, Austria, CW 40-14, 2023/2024

Data source for Figs. 1 and 2: Estimated ILI/100,000 inhabitants is an estimate based on data from the Flu Information System of Magistrate 15 of the City of Vienna and Dept. 7 of the City of Graz.

Data source for Fig. 3: Insurance data: eligible insureds from ÖOGKK: employed, PD, KBG

Data source for tables 4 and 5: Virological Sentinel Surveillance System (DINÖ): National Reference Laboratory for Influenza, Dep. of Virology; Med. University Vienna; Virological non-sentinel surveillance system: 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 Med. University of Graz, Stmk; Microbiology Laboratory & Joint Practice for Travel Medicine, IBK, Tyrol; Analyse BioLab GmbH, Elisabethinen Linz, Upper Austria; SALK Labor GmbH, Salzburg; Institute for Medical, Microbiology, and Hygiene at Klinikum Wels-Grieskirchen, Upper Austria.

Technical information

The Austrian Reference Center 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 and the influenza information system of Dept. 7 of the City of Graz (until the 2021/2022 season also the influenza surveillance system Greater Innsbruck). Since the beginning of 2012, ILI case data from the influenza information systems have been processed once a week by the Reference Center 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 pediatricians) of the influenza information systems of Magistrate 15 of the City of Vienna and Dept. 7 of the city of Graz (until 2021/2022 also the influenza surveillance system from 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 Center 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 physicians).

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 Center for Virology of the Medical University of Vienna. 98 sentinel physicians (= reporting physicians) send weekly nasopharyngeal swabs of ILI cases to the National Reference Laboratory for Influenza for testing. The weekly number of specimens tested for influenza and the number of those with influenza virus detection are sent by the National Reference Laboratory to the Reference Center 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, Microbiology 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 by 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 detects the emergence of new influenza virus variants and allows comparison with strains included 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.

Excess mortality

Since influenza is often not recognized or registered as a cause of death, it is international standard that influenza-related deaths are estimated by modeling. Such a model has been established in Austria in cooperation of the National Reference Center for Influenza Epidemiology of the AGES (Department of Data Science and Modeling) 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).

Since the beginning of 2020, SARS-CoV-2 has been circulating in addition to influenza during the winter months. Therefore, it was necessary to adapt the previously used model to avoid overestimation of influenza-associated excess mortality and to correct for the circulation of SARS-CoV-2 in addition to extreme temperatures. A basic description of the model is available from Nielsen et al [1] (in English). To account for differences in lethality of SARS-CoV-2 variants, different dominant variants (wild type, alpha, delta, omicron) were modeled separately.

The model now used differs substantially from the previously used FluMOMO model ([2]): While previously unaudited mortality data from Statistics Austria were used, the audited, weekly, publicly available deaths from Statistics Austria are now included in the model. These have been published monthly since the beginning of the SARS-CoV-2 pandemic. We have therefore also retrospectively updated the estimate of influenza-associated excess mortality for previous years. The current results are presented in Table 1.

Table 1: Estimated number of deaths associated with seasonal influenza (IA) including 95% confidence interval (CI) for the 2015/2016-2022/2023 seasons (both CW 40-KW 20 of the following year), Austria.
Season CW Estimated number of deaths associated with influenza (95% CI).
2015/2016 40-20 492 (207; 777)
2016/2017 40-20 4.939 (4.585; 5.292)
2017/2018 40-20 4.277 (3.920; 4.633)
2018/2019 40-20 2.022 (1.748; 2.296)
2019/2020 40-20 1.714 (1.431; 1.998)
2020/2021 40-20 0
2021/2022 40-20 652 (215; 1.089)
2022/2023 40-20 4.020 (3.578; 4.462)

Data sources

Deaths in Austria (excluding deaths abroad) as of 2000 by calendar week: Statistik Austria - from Statistik Austria).

Weekly influenza positive rate: virological sentinel surveillance system, DINÖ, Diagnostic Influenza Network Austria.

Weekly ILI incidence: clinical sentinel surveillance system, operated at the National Reference Center for Influenza Epidemiology, AGES.

National surveillance data for SARS-CoV-2

Temperature data for Austria: NOAA (National Oceanic and Atmospheric Administration) provided by EuroMOMO Network


1 Nielsen J, Rod NH, Vestergaard LS, Lange T. Estimates of mortality attributable to COVID-19: a statistical model for monitoring COVID-19 and seasonal influenza, Denmark, spring 2020. Eurosurveillance. 2021; doi:10.2807/1560-7917.ES.2021.26.8.2001646

2 Nielsen J, Krause TG, Mølbak K. Influenza-associated mortality determined from all-cause mortality, Denmark 2010/11-2016/17: The FluMOMO model. Influenza Other Respir Viruses. 2018; doi:10.1111/irv.12564


PCR tests, antigen detection by ELISA or rapid test, as well as virus culture and serological detection of antibodies are used for the diagnosis of influenza. In terms of sensitivity and specificity, PCR is considered the gold standard.


Nationale Referenzzentrale für Influenzaepidemiologie

Last updated: 11.04.2024

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