In most cases influenza is a benign illness but for elderly people, pregnant women and people with chronic diseases, the complications of flu can be dangerous. Vaccination remains the best way of preventing the complications of flu and hospitalisation.

Symptoms of influenza

The doctor will look to see if some or most of the symptoms of influenza are present. During an epidemic flu-type symptoms are sufficient to diagnose an influenza infection.

From the announcement that the influenza virus is in circulation, every influenza-type syndrome is probably influenza unless the contrary can be proved. However, this does not mean that the virus involved is identified.

Nose and throat swab

In doubtful cases, the doctor may take a nose and throat swab (respiratory sample) in the 7 days following the onset of the symptoms. This sample is then sent to a laboratory to diagnose the virus involved.

This step is recommended for persons at risk and where an antiviral treatment is envisaged.

Virological tests are also necessary for the monitoring of influenza during an epidemic to determine the viral strains in circulation and to update the vaccines.

Blood sample

A single blood sample to screen for antibodies is not effective in diagnosing influenza. Two samples must be taken at an interval of 2 weeks for definite confirmation of an increase in antibodies. The result therefore becomes available at the time of convalescence and so is of little interest to the patient.

The aim of serological diagnosis is to check the efficacy of the vaccine and to carry out epidemiological studies (scientific studies of diseases in a population according to different criteria: geographical, sex, how they occur and spread).


Sciensano coordinates a network of general practitioners and hospitals to ensure the permanent surveillance of influenza activity, of the intensity and severity of epidemics and the impact on the population. Sciensano is also the National Reference Centre (NRC) for influenza virus. The Belgian Official Medicines Control Laboratory (OMCL) of Sciensano, together with the European OMCL network, is responsible for the quality control of the influenza vaccine prior to marketing.

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