#17,833
Until a few years ago - with the notable exceptions of China, Indonesia, Egypt, Vietnam and Hong Kong - most of the world's nations avoided the use of poultry avian influenza vaccinations in favor of the long-standing OIE recommendation to `stamp out' HPAI infections by culling infected flocks.
That policy advised that vaccines should only be used as a temporary measure, stating that: `Any vaccination campaign must include an “exit strategy” i.e. a return to classic disease control measures.'
But as the HPAI H5 threat has spread globally, and the economic losses have continued to mount, some countries have expressed interest in using poultry vaccines to protect their flocks. A few countries, including France and Mexico, have already begun limited field use, while others (including the United States) are still evaluating the benefits and risks.
Although poultry vaccines can help protect flocks against HPAI H5, they must be properly applied and frequently updated, and even then they don't always offer complete protection.
Meaning that some birds may still become infected, but due to their vaccination status, they may not show signs of illness. This, many fear, could increase the chances of the virus spreading (to other birds, or potentially, even humans).
As we've seen many times in the past, the track record of using poultry vaccines to control avian influenza over the years has been mixed.
- One remarkable success, China's massive H5+H7 poultry vaccination program of 2017 shut down their H7N9 epizootic and the spillover of that deadly virus into humans. But HPAI H5N6 continues to spread (apparently asymptomatically) in poultry, occasionally infecting humans, often causing severe illness and sometimes death.
- But despite years of using poultry vaccines to control H9N2 in China, that LPAI virus continues to spread, evolve, and to spillover into humans (see J. Virus Erad.: Ineffective Control Of LPAI H9N2 By Inactivated Poultry Vaccines - China).
- Earlier this year, in WUR: 2 of 4 H5 Poultry Vaccines Tested Appear Effective Against H5N1, we saw a study out of the Netherlands that looked at 4 H5 poultry vaccines (2 currently in use, and 2 newer entries) and found the two currently in use were ineffective.
- In a 2014 EID Journal dispatch called Subclinical Highly Pathogenic Avian Influenza Virus Infection among Vaccinated Chickens, China the authors warned: `HPAI mass vaccination may increase spread of a virus that otherwise would be easily identified by observation of clinical signs.'
- And a 2012 study (see Egypt: A Paltry Poultry Vaccine), examined the effectiveness of six commercially available H5 poultry vaccines used in Egypt and found only one actually appeared to offer protection.
They also note that regions that have heavily employed poultry vaccines have also seen more rapid evolution of avian flu viruses.
Association of poultry vaccination with the interspecies transmission and molecular evolution of H5 subtype avian influenza virus
Bingying Li, Jayna Raghwani, Sarah C Hill, Sarah Francois, Noemie Lefrancq, Yilin Liang, Zengmiao Wang, Lu Dong, View ORCID ProfilePhilippe Lemey, Oliver G. Pybus, Huaiyu Tian
doi: https://doi.org/10.1101/2023.12.20.572711
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Abstract
The effectiveness of vaccinating poultry in preventing the transmission of highly pathogenic avian influenza viruses (AIVs) has been questioned for years and its impact on wild birds is uncertain (1-3). Here we reconstruct movements of H5 subtype AIV lineages among vaccinated poultry, unvaccinated poultry, and wild birds, worldwide from 1996 to 2023.
We find that lineage transitions among host types are lagged and that movements from wild birds to unvaccinated poultry were more frequent than those from wild birds to vaccinated poultry. However, we also find that the HA gene of the AIV lineage that circulated predominately among Chinese poultry with high vaccination coverage underwent faster evolution and greater nonsynonymous divergence than other lineages.
Further, this Chinese poultry lineage contained more codons inferred to be under positive selection, including at known antigenic sites, and its rates of nonsynonymous divergence and adaptative fixation increased after mass poultry vaccination began.
Our results indicate that the epidemiological, ecological and evolutionary consequences of widespread AIV vaccination in poultry may be linked in complex ways, and that much work is needed to better understand how such interventions may affect AIV transmission to, within and from wild birds.
(SNIP)
In conclusion, we find that vaccination in Asian poultry likely reduced the inter species transmission of these viruses. H5 AIV in Chinese poultry, which are highly vaccinated, show evidence of greater HA gene molecular evolution and adaptation after the introduction of vaccination.
Such circumstances may have increased the probability that birds susceptible to AIV belong to wild species at the interface between wild birds and poultry, leading to shifts in selection pressure on the virus. As avian influenza continues to pose significant challenges to wild and domestic animal health, our research can help inform the development of preventive measures against AIV, such as global vaccination policies.
(Continue . . .)
Many people understandably see poultry vaccines as the only practical tool to limit the economic losses from the global spread of HPAI, while others look back at the last 20+ years and wonder how much of our current avian flu problem has been exacerbated by suboptimal vaccine campaigns of the past.
As more countries decide to embrace AI poultry vaccination (as I suspect many will), the need for a better understanding of its impact - and potential risks - only increases.While it is seems reasonable that a properly applied, well-matched, and frequently updated poultry vaccination program can be an effective strategy against avian flu - at least in captive birds - history has shown those standards to be difficult to maintain.
