In September 2023 the WHO announced recommendations for the viral composition of influenza vaccines for the 2024 influenza season in the southern hemisphere. Issued at an information session at the Composition of Influenza Virus Vaccines meeting, the updates are based on consultations with experts from WHO Collaborating Centres and WHO Essential Regulatory Laboratories. These teams assess surveillance data generated by the Global Influenza Surveillance and Response System (GISRS). Their recommendations are used by national regulatory agencies and pharmaceutical companies to tailor influenza vaccines for the approaching season.  

What’s new? 

WHO emphasises that updating the viruses contained in influenza vaccines is important to ensure that they are effective as the viruses constantly evolve. So, what is the recommendation? 

For trivalent vaccines the following are recommended: 

  • An A/Victoria/4897/2022 (H1N1)pdm09-like virus 
  • An A/Thailand/8/2022 (H3N2)-like virus 
  • A B/Austria/1359417/2021 (B/Victoria lineage)-like virus 
Cell culture- or recombinant-based: 
  • An A/Wisconsin/67/2022 (H1N1)pdm09-like virus 
  • An A/Massachusetts/18/2022 (H3N2)-like virus 
  • A B/Austria/1359417/2021 (B/Victoria lineage)-like virus 

For quadrivalent egg- or cell culture-based or recombinant vaccines WHO recommends the inclusion of the B/Phuket/3073/2013 (B/Yamagata lineage)-like virus.  


The B/Yamagata family has “not been seen since March 2020”, reports STAT, and it is believed that measures taken in response to the COVID-19 pandemic may have forced it out of circulation. Not only is there “no benefit” to including this family in the vaccine, but the meeting report suggests that there is a “theoretical risk of reintroduction” if it is included. This risk “can be mitigated” by the removal of these viruses from the vaccines.  

“It is the opinion of the WHO influenza vaccine composition advisory committee that the inclusion of a B/Yamagata antigen as a component of influenza vaccines is no longer warranted, and every effort should be made to exclude it as soon as possible.”  

Following the apparent elimination of these viruses from circulation, discussion in the community has considered the possibility of “replacing that component” in quadrivalent vaccines with another type of flu virus, such as a bird or swine flu virus. Professor Kanta Subbarao, Director of the WHO Collaborating Centre for Reference and Research on Influenza, suggested that this would not be “very straightforward”.  

“I think there is a great interest from the influenza research community, but it is still at the research stage rather than being ready for the kinds of discussions that our committee has to make recommendations for the vaccine composition.”  

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