Credit NIAID
#19,070
In addition to deciding which influenza viruses to include in the next seasonal flu vaccine (see WHO Recommendations for Influenza Vaccine Composition for the 2026-2027 Northern Hemisphere Influenza Season), twice each year flu researchers are asked by WHO to advise on the development of new CVVs (Candidate Vaccine Viruses) for zoonotic influenza.While our attentions are primarily focused on the clade 2.3.4.4b H5N1 virus, there are literally dozens of other zoonotic influenza A viruses circulating around the world (see CDC IRAT list), all of which are believed to have some degree of pandemic potential.Each year new variants - subclades, subtypes, or genotypes - emerge, and the WHO must decide if they warrant the creation of a CVV. Since H5 viruses evolve rapidly, many older CVVs no longer match circulating strains.
Having a proven CVV already tested and approved can save valuable time if rapid production and distribution of a pandemic vaccine are ever required.
And over the past two decades, the WHO has recommended a lot of them (see list below). Forty-six are approved and available, and 6 are pending. And that's just for H5 subtypes.
They WHO has also approved 22 H7 CVVs (20 available, 2 pending), and 11 H9 CVVs (8 available, 3 pending), along with a smattering of other subtypes (H1, H3, H10, etc.).
This week the WHO published a 13-page update which contains background information on a wide range of novel avian and swine flu viruses detected over the past 6 months.
While they cite ongoing antigenic changes - particularly among the H5Nx panoply of viruses - in the end only one new CVV was recommended for development; a new A/Hunan/40087/2025-like H9N2 CVV.
We've been following concerning antigenic changes in H9N2 viruses for quite some time (see China CDC Weekly: Epidemiological and Genetic Characterization of Three H9N2 Viruses Causing Human Infections — Changsha City, Hunan Province, China, April 2025).
While no new H5 CVVs were recommended, antigenic characterization is still pending on a number of recent human H5 infections, including last September's H5N2 case from Mexico, and several recent H5N1 cases in Cambodia (clade 2.3.2.1e), along with a number of poultry isolates collected from Egypt, Laos, and Brazil.
This report also finds that the recent H5N5 virus isolated from a human in Washington State showed reduced reactivity to existing CVVs.The HA1 of the A(H5N5) virus from the human case in the United States of America had five amino acid substitutions relative to the A/Astrakhan/3212/2020 CVV, including a gain of a potential glycosylation site. Post-infection ferret antisera raised against clade 2.3.4.4b A/American Wigeon/South Carolina/22-000345-001/2021, A/Ezo red fox/Hokkaido/1/2022 and A/Astrakhan/3212/2020 CVVs showed reduced reactivity to the A(H5N5) virus.For now, however, the WHO has decided the threshold has not been met to justify recommending the creation of an H5N5 CVV.
I've only posted the summary, follow the link to read it in its entirety. I'll have a bit more after the break.
February 2026
The development of influenza candidate vaccine viruses (CVVs), coordinated by the World Health Organization(WHO), remains an essential component of the overall global strategy for influenza pandemic preparedness.
Selection and development of CVVs are the first steps towards timely vaccine production and do not imply a recommendation for initiating manufacture. National authorities may consider the use of one or more of these CVVs for pilot lot vaccine production, clinical trials and other pandemic preparedness purposes based on their assessment of public health risk and need.
Zoonotic influenza viruses continue to be identified and evolve both antigenically and genetically, leading to the need for additional CVVs for pandemic preparedness purposes. Changes in the antigenic and genetic characteristics of these viruses relative to existing CVVs and their potential risks to public health justify the need to develop new CVVs.
This document summarizes the antigenic and genetic characteristics of recent zoonotic influenza viruses and related viruses circulating in animals1 that are relevant to CVV updates. Institutions interested in receiving these CVVs should contact WHO at gisrs@who.int or the institutions listed in announcements published on the WHOwebsite2.
While having a closely-matched CVV might shave weeks or even months off the time it would take to develop, manufacture, and deploy a pandemic vaccine, there are still many obstacles to overcome.
Any flu jab would likely require 2 shots - 30 days apart - and even under the best case scenarios, it would likely take 6 months or longer before substantial quantities of vaccine are available to the public.
Some past blogs on some of the logistics of developing a pandemic vaccine include:
