#18,934
If there is any constant in the chaotic and highly mutable world of influenza A viruses, it is that these viral intruders are always evolving; either slowly - via antigenic drift - or more abruptly, through antigenic shift (reassortment).
It is why, in 2013 we saw the unexpected emergence of a deadly (in humans) LPAI H7N9 virus in China, and in 2014, the debut of an HPAI H5N8 virus in South Korea, which would eventually outshine H5N1 for the rest of the decade.
Over time, both viruses lost momentum. H7N9, in large part due to an emergency poultry vaccine introduced in 2017, and H5N8, due to the emergence of a `new and improved' H5N1 subtype in 2020.
A more recent example; 18 months ago we learned that the older 2.3.2.1c clade of H5N1 had reassorted with the newer clade 2.3.4.4b in the Mekong Delta, which has sparked > 2 dozen Cambodian human infections with a near 50% CFR. This new reassortment was subsequently reclassified as clade 2.3.2.1e.The point being, that anything we say about a given influenza A virus today is subject to change tomorrow.
While much of our attention is focused on HPAI H5 right now, we also keep one eye on lesser threats, including LPAI H9N2, which is not only ubiquitous in poultry across much of Asia and Africa, it has increasingly been reported in humans.
While H9N2 has a reputation of being a relatively mild viral infection - primarily reported in children - we've been changes in its epidemiology of late; including more adult infections (some seriously ill) reported in out of China.
H9N2 also easily reassorts with, and often enhances, other novel influenza viruses (including H7N9, H5N1, and H5N6), making it an important viral co-conspirator (see Vet. Sci.: The Multifaceted Zoonotic Risk of H9N2 Avian Influenza).
But, as a pandemic threat, LPAI H9N2 doesn't get a lot of respect.
It is a low-path virus in poultry, and not considered `reportable' to WOAH. Some countries vaccinate poultry against it, but existing vaccines have not been very effective (see (see J. Virus Erad.: Ineffective Control Of LPAI H9N2 By Inactivated Poultry Vaccines - China).
In China, where the bulk of human cases have been reported, we've seen renewed interest the past couple of years, as the virus has slowly evolved towards a more mammalian-adapted virus.
Last week, in China CDC Weekly: Epidemiological and Genetic Characterization of Three H9N2 Viruses Causing Human Infections, we looked at a local CDC investigation into 3 pediatric cases which were reported last April from Changsha City, Hunan Province, China.
Their report found a number of indicators of increased mammalian adaptation within the virus, including an enhanced ability to infect upper respiratory (α2,6-sialic acid) tract receptors, and a number of HA protein mutations, including; H191N, A198V, Q226L, and Q234L.
Which brings us to a new study, published yesterday in Emerging Microbes & Infections, which compares two H9N2 isolates (from 2024 and 1999) across several models, and finds today's virus far better adapted to human hosts.
While only two virus isolates were compared, and this study does not test transmission dynamics beyond cell and organoid infectivity, this is a reminder that even presumed `minor threats' like H9N2 have the potential to evolve into something more formidable.
I've posted the Abstract and a small excerpt, but you'll want to follow the link to read it in its entirety. I'll have a small postscript after the break.
Enhanced replication of a contemporary avian influenza A H9N2 virus in human respiratory organoids
Lin-Lei Chen, Jonathan Daniel Ip, Wan-Mui Chan, Stephanie Joy-Ann Lam,
Rhoda Cheuk-Ying Leung, Cyril Chik-Yan Yip, show all
Article: 2576574 | Received 15 May 2025, Accepted 14 Oct 2025, Accepted author version posted online: 16 Oct 2025, Published online: 03 Nov 2025
https://doi.org/10.1080/22221751.2025.2576574
ABSTRACT
H9N2 is currently the second most common avian influenza A virus subtype infecting humans. Monitoring viral phenotypic and genotypic adaptation to humans is crucial for risk assessment. Here, we compared the replication of an H9N2 human isolate collected in 2024 (A/HK/2346/2024) to a human isolate collected in 1999 (A/HK/1073/1999).
In Madin Darby canine kidney (MDCK) cells, A/HK/2346/2024 and A/HK/1073/1999 replicated to 8 and 5 log10 plaque-forming units (PFU) per ml, respectively. In both human nasal and lung organoids, A/HK/2346/2024 replicated to 6 log10 PFU/ml, but A/HK/1073/1999 failed to replicate in either organoid. The infection rates of both ciliated and non-ciliated cells and the ratios of infected 2,6/2,3 cells were higher for A/HK/2346/2024 than A/HK/1073/1999.
Apart from the mammalian adaptive substitutions that were present in the nasopharyngeal specimen collected on day 1 post-symptom onset (pso) (HA-D183N/D190 T/Q192R/Q226L; NA-del62-64; PB2-A588V/K702R; PB1-I368V; PA-K356R/S409N; M1-R95K), the mammalian-adaptive substitution PB2-D253N emerged de novo on day 7 pso.
Analysis of all human (n = 96) and avian influenza (n = 14,762) H9N2 deposited at GISAID showed the dominance of several human-adaptive substitutions in H9N2 strains collected from humans in recent years.
In summary, we demonstrated that a recent H9N2 virus is more adapted to humans, and is able to replicate to high titres in both upper and lower human respiratory tract which may confer higher person-to-person transmissibility and virulence. Our study underscores the importance of human organoid-based phenotypic monitoring and inter/intrahost genotypic monitoring for assessing the zoonotic risk of avian influenza viruses.
(SNIP)
While H9N2 remains far from our biggest pandemic concern, the CDC has designated 2 different lineages (A(H9N2) G1 and A(H9N2) Y280) as having some pandemic potential (see CDC IRAT SCORE), and several candidate vaccines have been developed.H9N2 has been relatively neglected as a potential pandemic agent. Over the years, H9N2 has already acquired numerous mutations that facilitate adaptation to humans [35].
In this study, we demonstrated the enhanced fitness of the contemporary H9N2 isolate in human nasal organoid and identified the emergence of human adaptive mutations that only appeared late during infection. Being a reservoir of internal genes for AIVs that cause severe human infections, H9N2 can significantly contribute to an AIV that causes a human pandemic. Having a low pathogenicity in birds, H9N2 can spread widely in wild birds without being noticed. In fact, H9N2 is the predominant AIV subtype detected among poultry [19].
Antigenic drift has allowed the virus to evade from vaccine-induced immunity [49]. With widespread co-circulation of H5N1 2.3.3.4b and H9N2 in wild birds [50], there is an increased chance of reassortment between these two strains. If an H5N1 2.3.4.4b, which already circulates widely in mammals, reassorts with an H9 virus, there is a high risk of an endemic or even a pandemic virus. Continual genotypic and phenotypic surveillance of H9N2, especially from human and mammalian infections, plays a pivotal role in the assessment of zoonotic risk of all AIVs [51].
(Continue . . . )
And many will be surprised to see that, in terms of risk of emergence, the H9N2 Y280 lineage is ranked higher than H5N1, while the G1 lineage is ranked only slightly lower.
Of course, something that is currently not even on our radar could emerge as the next big threat. Afterall, no one saw SARS-CoV-2 as a threat until it was already upon us, and H7N9 emerged with no warning.
The only real questions are when the next pandemic will emerge, how bad it will be, and whether we'll be ready when it does.
The first two are out of our control. As for the third, we seem even less prepared today, than we were before the last pandemic hit.
