Influenza epidemics occur every winter with high impact on disease burden, hospitalisations and excess mortality in the countries of Europe. To understand the characteristics of circulating influenza viruses during seasonal epidemics, virological influenza surveillance is performed, and detected viruses are further characterised at national influenza centres (NICs) that are part of the wider network of the WHO Global Influenza Surveillance and Response System (GISRS). External quality assessment (EQA) is an important instrument in assessing the quality of the generated data that are reported nationally and internationally through The European Surveillance System (TESSy) and presented in The European Respiratory Virus Surveillance Summary (ERVISS), the joint European Centre for Disease Prevention and Control (ECDC) and World Health Organization (WHO) Regional Office for Europe weekly update on respiratory virus infections (https://erviss.org/). From March to July 2023, a European External Influenza (virus) Quality Assessment Programme exercise (EEIQAP) was held for NICs and other national influenza reference laboratories in the European Union/European Economic Area (EU/EEA) and other countries in the WHO European Region. The exercise covered influenza virus molecular detection, isolation, strain genetic and antigenic characterisation and antiviral susceptibility testing. This was the eighth detection, isolation and strain characterisation panel and the sixth antiviral susceptibility testing panel since the start of the scheme in 2000. Since EEIQAP 2010, all exercises have been organised with the support of the European Centre for Disease Prevention and Control (ECDC) and since EEIQAP 2018, with the additional support of the WHO Regional Office for Europe. The EEIQAP 2023 was organised by the contractor, the National Institute for Public Health and the Environment (RIVM) in Bilthoven, the Netherlands, for the European Reference Laboratory Network for Human Influenza (ERLI-Net). Participation of laboratories from the EU/EEA, Western Balkan countries and Türkiye was supported by ECDC. Participation in EEIQAP 2023 by laboratories in additional European countries was supported by the WHO Regional Office for Europe. For the third time, results are jointly presented for the EU/EEA together with other participating laboratories in the WHO European Region. The objectives of the exercise were to collect information on the capacity and capability of the network regarding i) rapid molecular influenza virus detection, A/B typing, type A H- and N-subtyping and B lineage determination by reverse transcription polymerase chain reaction (RT-PCR), ii) influenza virus isolation and strain characterisation using antigenic and/or genetic techniques, and iii) antiviral susceptibility testing using genetic and/or phenotypic techniques, within a defined reporting timeframe of seven working days for i) and 36 working days for ii) and iii). This exercise aimed to provide the participants with an independent assessment of their own laboratories' performance and a comparison with other reference laboratories for influenza in the European Region. In addition, performance in the EEIQAP 2023 provided a validity check of the data reported to TESSy. For the first time in 2023, the programme included genetic testing for reduced susceptibility markers to the antiviral drug baloxavir marboxil (BXM). Reporting of the sequence data obtained for the hemagglutinin (HA), neuraminidase (NA) and acidic polymerase (PA) coding genome segments was included in conjunction with reported genetic characterisation data.
