But it is not without good reason, for H9N2 has shared its internal genes with - and thereby helped to create - some of the most dangerous avian flu viruses (H5N1, H7N9, H10N8) on the planet. A few (of dozens) or studies include:
|Range Of Endemic H9N2 Viruses|
This global expansion comes despite massive efforts in China, Egypt, and a few other other countries to control the virus via poultry vaccination. We've seen those poultry vaccination programs loose effectiveness over time however (see Egypt: A Paltry Poultry Vaccine and The HPAI Poultry Vaccine Dilemma), as the virus mutates and vaccines are not updated or uniformly deployed.
All of this has ramifications beyond H9N2, as we've recently seen the resurgence of a far more dangerous HPAI H5N6 in China, three years after a very successful nationwide deployment of a new H5+H7 poultry vaccine.
Three years ago, in EID Journal: Two H9N2 Studies Of Note, we looked at two reports which suggest that H9N2 continues to evolve away from current (pre-pandemic and poultry) vaccines and is potentially on a path towards better adaptation to human hosts.
The failure of vaccination might be because of inefficient application, low dose, and low vaccination coverage (especially in the household sector).11,12 Moreover, the continuing transmission in combination with the intensive long-term usage of the inactivated virus vaccine may have led to antigenic changes leading to immune escape.
The link, and excerpts from the open access study follow.
Vaccination with inactivated virus against low pathogenic avian influenza subtype H9N2 does not prevent virus transmission in chickensHongrui Cui ab Mart CMde Jongb NancyBeerensc Monique M.van Oersd Qiaoyang Tenga Luzhao Lia XuesongLia QinfangLiua ZejunLiahttps://doi.org/10.1016/j.jve.2021.100055Get rights and content
Under a Creative Commons license
H9N2 subtype avian influenza has spread dramatically in China ever since first reported in the 1990s. A national vaccination program for poultry was initiated in 1998. Field isolation data show that the widely used inactivated H9N2 vaccine does not provide effective control of the transmission of this low pathogenic avian influenza (LPAI) virus in poultry.Current research has focused on two reasons: (i) insufficient immune response triggered by the vaccination with the inactivated virus, (ii) the occurrence of escape mutants selected by vaccine-induced immune pressure. However, the lack of effectivity of the inactivated virus vaccine to sufficiently reduce transmission has been noticed.We mimicked the natural infection and transmission process of the H9N2 virus in vaccinated and non-vaccinated chickens. A statistical model was used to estimate the transmission rate parameters among vaccinated chickens, varying in serum hemagglutinin inhibition titers (HIT) and non-vaccinated chickens. We demonstrate, for the first time, that the transmission is not sufficiently reduced by the H9N2 vaccine, even when vaccinated chickens have an IgG serum titer (HIT>23), which is considered protective for vaccination against homologous highly pathogenic avian influenza (HPAI) virus.Our study does, on the other hand, cast new light on virus transmission and immune escape of LPAI H9N2 virus in vaccinated chickens populations, and shows that new mitigation strategies against LPAI viruses in poultry are needed.
In our research combining an experimental transmission study with mathematical modeling, we show that inactivated H9N2 vaccine is able to provide individual protection, but unable to stop the transmission of the H9N2 virus in chickens. The power of the statistical test used was 78%. Transmission modeling can give a statistical estimation of the vaccine effects on virus transmission in certain populations, thereby providing guidance for control strategies.
While poultry vaccination programs such as we've seen in China have saved countless millions of birds from being culled and in many cases - have reaped tremendous benefits - unless vaccines are frequently updated to match current viruses - and are correctly and universally applied - they can lose their effectiveness and eventually become a double-edged sword.