EU Recommendations for the Seasonal Influenza Vaccine Composition for the Season 2023/2024

messages in brief | 15/05/2023

Currently, only sporadic cases of influenza virus infections are detected in Austria (flu wave officially ended in week 14 2023). During season 2022/23, influenza activity returned to levels typical of pre-COVID-19 pandemic years with influenza A H3N2 viruses predominating. In the second half of the season Influenza B viruses circulated at moderate levels (all analyzed viruses belonged to B/Victoria lineage). Worldwide, there have been no confirmed detections of circulating B/Yamagata lineage viruses after March 2020.

Influenza viruses can be fatal, but preventive vaccination is a suggestive option.

The recommendations for the seasonal influenza vaccine composition have to be re-evaluated each year since influenza viruses are subject to a considerable antigenic variability and new viruses emerge.
Each year in March, European experts decide on the new composition based on recommendations published by the WHO. The timely recommendation allows manufacturers and regulators sufficient time to ensure availability of vaccines prior to start of the next influenza season.

The decision on which influenza or vaccine strains should be included in the seasonal influenza vaccines is based on epidemiologic and clinical findings from the Global Influenza Surveillance and Response System (GISRS) network: typically, new viruses emerge in Asia and spread via Europe towards the Americas and the southern hemisphere. For the purpose of virus surveillance, the GISRS including 149 National Influenza Centres in 125 WHO Member States collected and tested numerous clinical specimens in the last year.

Beyond GISRS data, additional data are considered to ensure that the recommended strains match as closely as possible the strains circulating next season thereby contributing to vaccine effectiveness: serological and genetic characterization studies of recently circulating viruses, virus fitness forecasting, and vaccine effectiveness data.

Taking into account the above information, quadrivalent vaccines in season 2023/2024 should contain:

Egg-based Vaccines:

  • an A/Victoria/4897/2022 (H1N1)pdm09-like virus
  • an A/Darwin/9/2021 (H3N2)-like virus
  • a B/Austria/1359417/2021 (B/Victoria lineage)-like virus
  • a B/Phuket/3073/2013 (B/Yamagata lineage)-like virus

Cell culture-based vaccines:

  • an A/Wisconsin/67/2022 (H1N1)pdm09-like virus
  • an A/Darwin/6/2021 (H3N2)-like virus
  • a B/Austria/1359417/2021 (B/Victoria lineage)-like virus
  • a B/Phuket/3073/2013 (B/Yamagata lineage)-like virus

Trivalent vaccines should contain the following components: influenza A H1N1-pdm09, influenza A H3N2, and influenza B (Victoria lineage).

Compared to the previous season 2022/2023, the above recommendation differs with respect to the recommendation for influenza A H1N1-pdm09 viruses. Since the same strains are not optimal for both production in egg-based and cell-based production systems, different viruses are recommended for the influenza A H1N1-pdm09 and influenza A H3N2 components. This facilitates timely vaccine production.

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Further inquiry note