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.

worldwide

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.

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

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.

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.

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.

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.

Data sources

Deaths in Austria (excluding deaths abroad) as of 2000 by calendar week: Statistik Austria - data.statistik.gv.at(open.data 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

References

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