A multi-year Dutch field study has found that vaccinating laying hens against highly pathogenic avian influenza can significantly reduce virus transmission, mortality and the likelihood of outbreaks, while maintaining commercial production performance.

The study, carried out by Wageningen University & Research (WUR) with Royal GD and Utrecht University, examined the performance of HVT-based H5 vaccination under Dutch commercial farming conditions. It was commissioned by the Dutch Ministry of Agriculture, Fisheries, Food Security and Nature and funded by the ministry together with the Dutch poultry sector, represented by AVINED.

The work monitored vaccinated laying hens for more than 85 weeks, covering the full production cycle. The wider research project investigated two commercial vaccines, VAXXITEK HVT+IBD+H5 and Vectormune AI, while the linked full report covers VAXXITEK HVT+IBD+H5, including a prime-and-booster strategy using Volvac B.E.S.T. AI+ND.

Transmission experiments at Wageningen Bioveterinary Research showed that one unvaccinated adult laying hen could infect, on average, more than six other birds in a fully susceptible flock. In vaccinated hens, virus transmission was significantly slower, with the reduction in spread strongly linked to antibody levels within the flock. Groups with higher and more uniform antibody titres showed lower transmission rates.

Flock-level modelling indicated that, without vaccination, the probability of an outbreak after virus introduction was about 96%. In vaccinated flocks, that fell to around 10-30%, depending on the vaccination strategy and whether a booster was used. The report also found that vaccination reduced the median epidemic size from 14 farms without vaccination to one or two farms under vaccinated scenarios.

Reduced likelihood of an outbreak

WBVR researcher Kim Bouwman, project leader of the study, said: “The results show that vaccination substantially reduces the likelihood of an outbreak.”

The researchers said maintaining high and stable antibody levels was critical. Monthly sampling showed antibody levels increased during rear and remained high through lay, but some fluctuation was seen in flocks that received only a primary vaccination. Flocks receiving a booster maintained consistently high antibody levels throughout the production cycle.

The study also found vaccination to be safe for commercial layer production. No visible side effects were observed, flock health remained stable and mortality stayed within normal commercial ranges. Egg production also reached normal levels, with no negative effects identified on production performance or animal welfare.

Surveillance essential

However, WUR stressed that vaccination should not be viewed as a standalone control measure. Because vaccination reduces clinical signs and mortality, outbreaks may become less visible, making active surveillance essential. The researchers said passive surveillance alone would detect only a limited proportion of outbreaks in vaccinated flocks, while an appropriate monitoring system could detect more than 93% in time.

Diagnostic monitoring remained effective during the study. More than 8,600 blood samples were analysed, with 99.8% testing negative, indicating no virus circulation. PCR testing of samples from dead birds also supported the absence of field infection during the trial.

The safety of poultry products was also assessed. The researchers found only occasional PCR-positive results on eggshells or in tissues, without infectious virus being isolated, and concluded that the risk to egg and poultry product safety was minimal.

The findings provide further evidence that vaccination could form part of a more sustainable HPAI control strategy for the poultry sector, provided it is combined with active monitoring, rapid response systems and existing biosecurity measures.

Read the full research report: Vaccination of poultry with HVT-based H5 vaccine against highly pathogenic avian influenza H5N1 virus.

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