WHO: Candidate Vaccine Viruses for Pandemic Preparedness - Feb 2025

Credit NIAID

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In addition to deciding which influenza virus components to include in the seasonal flu vaccine, twice each year flu researchers meet with the WHO to discuss the development of new CVVs (Candidate Vaccine Viruses) for zoonotic influenza.

While our attentions are currently 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 have some degree of pandemic potential. 

Each year new variants - subclades, subtypes, or genotypes - emerge, and the WHO has to decide if they warrant the creation of a CVV.  Since H5 viruses evolve so rapidly, early attempts to stockpile large quantities of vaccine have pretty much been abandoned. 

Most of those early stockpiles have long since expired, or no longer match circulating strains.

Having a proven CVV already tested and approved can save months of valuable time if mass production and distribution of a pandemic vaccine are ever required, even though an emerging pandemic virus might not be an exact match.

Over the past 2 decades more than 4 dozen H5Nx candidate vaccine viruses (CVVs) have been selected by WHO for development. Many of these older CVVs are for viruses that no longer circulate in the wild, having been supplanted by newer versions.

The full document - which runs 14 pages - contains background information on a wide range of novel avian and swine flu viruses, but in the end only two new CVVs were recommended for development. 

•  A/Victoria/149/2024, an H5N1 virus isolated from a child returning to Australia from India (see EID Journal: Influenza A(H5N1) Virus Clade 2.3.2.1a in Traveler Returning to Australia from India, 2024) which we now know to be a new genotype, with contributions from the older clade 2.3.2.1a virus and newer clade 2.3.4.4b viruses.

• And A/Fujian/2/2024, an H5N6 virus belonging to subclade 2.3.4.4h, which raised concerns a year ago when two cases appeared in Fujian Province (see China CDC Weekly: Infection Tracing and Virus Genomic Analysis of Two Cases of Human Infection with Avian Influenza A(H5N6) — Fujian Province, China).

The full report is worth reading, as it provides a pretty good overview of the panoply of zoonotic influenza A viruses reported over the past year, including sporadic swine variant infections and  a noticeable uptick in H9N2 infections. 

As important as having a safe and effective vaccine will be during any severe flu pandemic, it is unlikely to be widely available until months into any outbreak (see Maggie Fox's SCI AM - A Bird Flu Vaccine Might Come Too Late to Save Us from H5N1).

Which is why NPIs (non-pharmaceutical interventions) - face masks, handwashing, social distancing, etc. - will be our primary tools against any novel respiratory virus. 

And why you might want to secure a reasonable quantity of N95 masks now for you and your loved ones, while prices are low and supplies are abundant.