Human vaccination for highly pathogenic avian influenza

The case of avian influenza A (H5) confirmed by the Centers for Disease Control and Prevention in Missouri, USA, on Sept 6, 2024,1 has reignited concerns about the risk of human contagion and the need for continued vigilance regarding avian influenza and its global public health implications. Vaccination against highly pathogenic avian influenza (HPAI) in humans is a topic of growing interest, given the potential pandemic threat posed by strains such as H5N12 and H7N9. These viruses are known for their high virulence and their ability to infect humans through direct contact with infected poultry,3 with mortality rates as high as 52%. Although interhuman transmission of the virus has been rare so far, some cases of prolonged transmission between people in Indonesia and Thailand have raised concerns about a potential global health crisis.4,5

 

Despite the implementation of control measures such as surveillance, biosecurity, and culling of infected animals, the virus continues to circulate among poultry. Some countries, such as Mexico, France, and China, have adopted the vaccination of animals as a preventive strategy, achieving a reduction in infection rates and viral load.6 However, to date, there is no mass vaccination programme for humans. Only in Finland has vaccination been initiated for high-risk groups, such as poultry and fur farm workers, opening the debate on the possibility of extending vaccination to the general population as well.7

 

Pre-emptively vaccinating humans against HPAI could substantially reduce the risk of the virus genetically scrambling with seasonal influenza strains, preventing the emergence of human-adapted variants capable of transmission between people. However, developing a universal vaccine effective against all variants of the virus remains complex, given the high rate of genetic recombination that characterises HPAI. So far, only the H5N1 and H7N9 strains have caused human infections, which limits the number of vaccine candidates to be developed.

 

Research on a vaccine against HPAI has accelerated, in part due to the use of technologies already used for vaccines against COVID-19, such as mRNA vaccines.8 This technology has proven to be effective in rapid vaccine development and could also be used to create vaccines that protect against both seasonal influenza and HPAI, thus reducing the need to develop multiple separate vaccines. In addition, vaccine technology already used for poultry could be adapted for randomised clinical trials to evaluate vaccine efficacy and safety for human use.

 

Several crucial questions have yet to be answered, however: what vaccine coverage will be needed to achieve herd immunity? Will vaccine-induced antibodies be sufficient to protect against an influenza pandemic? And will the vaccine offer cross-immunity against virus variants? In addition, the economic sustainability of such programmes will be a determining factor, especially in low-income and middle-income countries, where vaccine availability and adherence are often limited.

 

Even if a safe and effective vaccine were to be developed, vaccination alone will not be enough to control HPAI in humans. Vaccination will need to be supplemented with additional control measures, such as maintaining strict hygiene practices on farms, enhanced surveillance, and sharing real-time epidemiological data. This approach will enable the early detection of outbreaks and rapid response to contain them before the virus spreads widely. Implementing stringent biosecurity measures and promoting international collaboration will be essential to reduce the global risk of HPAI and protect public health.

Although vaccination represents a promising strategy for reducing the health and economic risk associated with HPAI, it is crucial that it be part of a broader set of global surveillance, prevention, and collaboration interventions.