HPAI Vaccination in Poultry: Field Relevance, Efficacy, and Limitations

Highly pathogenic avian influenza (HPAI) remains a major concern in poultry production, and vaccination is increasingly considered as part of control strategies. HPAI vaccines have an efficacy against mortality ranging from 78% to 97%, depending on vaccine platforms and the match (or mismatch) between vaccine strains and challenge strains¹. This protective effect is particularly marked when the vaccine strain and the challenging strain belong to the same clade, especially in inactivated and commercial vaccines¹. 

Vaccine Efficacy and Field Implications 

Vaccinated chickens challenged with HPAI virus may still transmit infection within flocks, a phenomenon often referred to as “silent spread”¹. Vaccine efficacy is not absolute, and protection is not guaranteed across all conditions¹. 

Excess mortality in vaccinated flocks may still be detectable and can exceed normal weekly mortality rates, including less than 1% in broilers². In some situations, mortality of around 3% has been observed even with vaccination under optimal conditions¹. 

These findings indicate that vaccination alone does not eliminate disease impact and that continued monitoring of flock health remains essential¹. 

Impact of Viral Evolution on Vaccine Performance 

Vaccine efficacy varies depending on the similarity between vaccine strains and circulating field strains¹. A higher level of protection is observed against homologous strains compared to heterologous strains¹. The rapid evolution of HPAI viruses can reduce vaccine effectiveness over time¹.  

This emphasises the importance of continuous epidemiological surveillance and periodic updating of vaccines¹. Recombinant vaccines may be less affected by such genetic variation, although further evaluation is required¹. 

Onset of Immunity and Vaccination Timing 

Protection following vaccination develops over time, and while some level of immunity may be observed earlier, optimal immune response is generally achieved after an adequate post-vaccination interval¹. 

Further research is needed to better understand early immune responses and the potential role of vaccination in emergency outbreak situations¹. 

Comparison with Previous Evidence 

Previous systematic reviews have explored the relationship between survival outcomes and antigenic similarity between vaccine and challenge strains¹. However, some analyses did not quantify the absolute reduction in mortality associated with vaccination². 

Recent evaluations provide a more structured understanding of vaccine performance, particularly in relation to mortality reduction¹. 

Limitations of Current Evidence 

Most available data are derived from studies conducted in specific-pathogen-free chickens rather than commercial broilers or layers, which may limit direct applicability to field conditions¹. 

In addition, the absence of maternal antibodies and variations in study design, including vaccination schedules and challenge protocols, contribute to variability in reported outcomes¹. 

Conclusion 

HPAI vaccination can reduce mortality but does not provide complete protection or prevent transmission¹. The potential for continued infection within vaccinated flocks, along with viral evolution, highlights the need for an integrated approach combining vaccination with surveillance and biosecurity¹. 

Ongoing evaluation and adaptation of vaccination strategies remain essential for effective control of HPAI in poultry¹. 

Reference  

1. Tseng I, Pan BY, Feng YC, Fang CT. Re-evaluating efficacy of vaccines against highly pathogenic avian influenza virus in poultry: A systematic review and meta-analysis. One Health. 2024 Jun 1;18:100714. https://doi.org/10.1016/j.onehlt.2024.100714  

2. Mo J, Spackman E, Swayne DE. Prediction of highly pathogenic avian influenza vaccine efficacy in chickens by comparison of in vitro and in vivo data: A meta-analysis and systematic review. Vaccine. 2023 Aug 31;41(38):5507-17. https://www.sciencedirect.com/science/article/am/pii/S0264410X23009222