Surveillance of Seasonal Influenza Viruses in Switzerland

In Switzerland, epidemiological and virological surveillance of the influenza virus (flu) is provided by the Sentinella reporting system (1986) and the mandatory reporting system. A third system, targeting the monitoring of influenza in hospitalized patients, has also been in the testing phase since 2018.
Of the primary care physicians (general practitioners, internists and pediatricians) participating in the Sentinella reporting system, around a half address nasopharyngeal swabs taken from some of their patients with influenza-like illness (ILI)(fever >38°C, and coughing or sore throat in the absence of another diagnosis) to the National Reference Center of Influenza (NRCI) during each influenza season (week 40 from one year to week 16 of the following year). The latter performs the diagnosis for influenza A and B. For samples that are positive for influenza A and/or B, a detailed characterization of the influenza viruses present in the sample will be performed (subtyping, antigenic proximity to vaccine strains, genotyping and identification of  mutations associated with antiviral resistance).
On this page, you will find a report of the number of samples tested by the NRCI; as well as the results relative to viral characterization.
We invite you to consult the website of the Federal Office of Public Health, if you are interested in an overview of the current situation relative to the epidemiology of influenza-like illness cases in Switzerland. The annual epidemic threshold is of 69 cases of influenza illness per 100'000 inhabitants. It was exceeded on week 02/2020.

Report on the activity of influenza viruses (NRCI data)

Week 02/2020
Last update 15 January 2020 
During week 02/2020, 52 samples were tested by the NRCI. Seventen were positive for influenza. Since week 40/19, a total of 258 samples have been tested. (Table 1 and charts 1-3).
Table 1. Report of influenza virus activity in the Sentinella population

Chart 1: Overview of the activity of human influenza viruses in the Sentinella population and incidence (‰) of influenza-like illness in the Swiss population
Chart 2: Percent positive and negative samples for influenza A and B. Since week 40/19 (left). During the week 02/2020 (right).
Of the 50 influenza  viruses tested since week 40/19, 21 were identified as A(H1N1)pdm09 strains and 8 as A (H3N2) strains; 14 Influenza B virus were also detected among which 10 bolonged to the Victoria-lineage (Charts 1 and 3). 
Chart 3: Number of negative and positive influenza A subtypes / B lineages since week 40/19
Influenza viruses characterisation
The 13 A(H1N1) pdm09 viruses tested are well recognized by the reference antiserum produced against the vaccine strain A/ Brisbane/02/2018. Similarly, the only A(H3N2) virus tested  shows good reactivity with the reference antiserum against the vaccine strain A/ Kansas/14/2017 (Table 2).
The 8 influenza B viruses tested are recognized by the antiserum produced against the vaccine strain B/Colorado/06/2017.
The antigenic proximity is determined on the basis of the haemagglutination inhibition test. The viruses and antisera (ferret) reference are provided by the Worldwide Influenza Center in London (Francis Crick Institute). Yamagata B strain is not present in trivalent influenza vaccines.
Table 2. Antigenic proximity between CNRI-characterized viruses and vaccine strains recommended by the World Health Organization for the 2019/2020 season
Reduced influenza vaccine efficacy against influenza A(H3N2) viruses has been demonstrated during several consecutive seasons. This could be explained to some extent by:
Influenza A(H3N2) viruses are more variable than influenza A(H1N1)pdm09 and B viruses. Each modification/mutation in a virus is likely to cause differences between the vaccine strain and the influenza viruses circulating during the season. These are antigenic modifications; and for yet unknown reasons, they are more frequent in influenza A(H3N2) viruses than in the other two strains. From the time the influenza vaccine strain is recommended to the actual vaccination event (more than 6 months delay), A(H3N2) viruses are more likely to have undergone mutations, which may result in an antigenic drift potentialy impacting the vaccine effectiveness.
The main mode of production of vaccine strains remains their amplification in chicken eggs. But while all influenza viruses undergo mutations induced by this mode of production, changes in influenza A(H3N2) viruses have an increased tendency to result in antigenic modifications than those of A(H1N1)pdm09 and B viruses. Other production methods exist but are still not widely used.
Contact :
For further information on the activity of human influenza viruses in Switzerland and on the different tests and methods used to characterize them contact Ana Rita Gonçalves, 022 372 40 81 ou
Useful links (accessed 15.01.2020):
Valuable information on the vaccine and influenza vaccination in Switzerland is available on the website
Centers for Disease Control and Prevention:


Last update : 15/01/2020