Friday, February 24, 2023

WHO Recommended 2023-2024 Northern Hemisphere Flu Vaccine Composition


Twice each year international influenza experts meet to discuss recent developments in human and animal influenza viruses around the world, and to decide on the composition of the next influenza season’s flu vaccine. Due to the time it takes to manufacture and distribute a vaccine, decisions on which strains to include must be made six months in advance.

Which means the composition of the northern hemisphere’s vaccine must be decided upon in February of each year, while decisions on the southern hemisphere’ vaccine are made in September.

Today the WHO announced the recommended vaccine composition for this fall's flu vaccine, along with recommendations for a new H5N1 candidate vaccine virus (CVV).

The WHO recommends that trivalent vaccines for use in the 2023-2024 northern hemisphere influenza season contain the following:

Egg-based vaccines

  • an A/Victoria/4897/2022 (H1N1)pdm09-like virus;
  • an A/Darwin/9/2021 (H3N2)-like virus; and
  • a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.
Cell culture- or recombinant-based vaccines

  • an A/Wisconsin/67/2022 (H1N1)pdm09-like virus;
  • an A/Darwin/6/2021 (H3N2)-like virus; and
  • a B/Austria/1359417/2021 (B/Victoria lineage)-like virus
For quadrivalent egg- or cell culture-based or recombinant vaccines for use in the 2023-2024 northern hemisphere influenza season, the WHO recommends inclusion of the following as the B/Yamagata lineage component:
  • a B/Phuket/3073/2013 (B/Yamagata lineage)-like virus.


Of note: the H1N1 component has been changed from the A/Sydney/5/2021-like viruses that were recommended for the 2023 southern hemisphere last September to an A/Wisconsin/67/2022-like virus for cell-based production and A/Victoria/4897/2022-like virus for egg-based production.

In a separate document (see Genetic and antigenic characteristics of zoonotic influenza A viruses anddevelopment of candidate vaccine viruses for pandemic preparedness February 2023), the WHO described recent findings of influenza (H5N1, H9N2, and H1N1) in animals around the world, and their recommendations for new vaccine development.

As we discussed two weeks ago, in How The WHO & CDC Are Developing Candidate H5N1 Vaccinessince H5N1 reemerged in 2003, more than three dozen H5 CVVs have been selected by WHO for development. Most of these viruses have gone extinct over time, while new ones have emerged, necessitating the continual development of new vaccine candidates. 

The WHO describes the recent spate of human H5 infections as follows:

Genetic and antigenic characteristics of influenza A(H5) viruses
Three A(H5N6) and two A(H5N1) infections were identified in China, one A(H5N1) infection in Ecuador, two A(H5N1) detections in Spain, and one A(H5N1) infection in Viet Nam. The majority of cases reported exposure to poultry. Two of the A(H5N6) cases were fatal, the third was severe. 
One of the A(H5N1) cases was fatal, three more were severe, and two were asymptomatic. All cases from which sequence information is available (n=6) were caused by clade viruses. The HAs of the sequenced viruses had 2 to 7 amino acid substitutions compared with the HA of A/Astrakhan/3212/2020, from which a clade CVV has been developed.

They report specifically on the clade viruses as:

Clade viruses were detected in birds in many countries in Africa, Asia, Europe, North America and, for the first time, in Central and South America. An increasing number of infections in wild and captive mammals has been reported, with mink-to-mink transmission suspected on a farm in Spain.

Viruses from this clade have been associated with several different NA subtypes with N1 now predominating. The high levels of infection in birds with these viruses and increased geographic distribution have been accompanied by genetic diversification.

Some A(H5N1) viruses from Europe, the United States of America (USA) and Viet Nam show reduced reactivity with post-infection ferret antisera raised against the A/Astrakhan/3212/2020 CVV.

All viruses from Europe reacted well with post-infection ferret antiserum raised against A/chicken/Ghana/AVL763_21VIR7050-39/2021; a representative CVV is being developed. Many viruses from the USA that had reduced reactivity with antisera raised against the A/Astrakhan/3212/2020 CVV showed better reactivity with post-infection ferret antisera raised against A/American wigeon/South Carolina/22-000345-001/2021 (Table 2). 

Based on these findings, they recommend:

Based on current antigenic, genetic and epidemiologic data, a new clade CVV that is antigenically like A/American wigeon/South Carolina/22-000345-001/2021 is proposed.

This document also describes recent human infections with H1 viruses, and H9N2, but does not recommend any new CVVs at this time.