These results, published in June 2022 in the scientific journal Viruses, provide grounds for a review of the vaccination strategy for chickens, as well as better anticipation of the consequences of vaccination against coronaviruses in general, and SARS-CoV-2 in humans in particular. Dr Paul Brown, study leader and scientist in the ANSES laboratory's Unit for virology, immunology and parasitology in poultry and rabbits (VIPAC), provides some explanations.
What is avian infectious bronchitis virus (IBV) ?
Paul Brown : It is a virus affecting poultry, principally hens, and mainly causes respiratory symptoms, but also kidney and genital problems, depending on the strain. IBV is now the second most important respiratory virus in chickens in terms of economic losses to the sector.
As IBV is a member of the coronavirus family, our results are not only of interest for animal health, but may also offer important lessons for human health.
Why did you decide to look at vaccination against this virus ?
PB : Vaccination against infectious bronchitis virus began in the 1950s. Although it has been carried out systematically in chicken farms since then, we have seen that IBV is still evolving. The assumption put forward by the scientific community is that intensive vaccination may contribute to the virus's evolution. However, before now, no specific studies had been performed to document this assumption.
We therefore used new bioinformatics tools to look into the subject, working with scientists from several units of ANSES's Plouzané-Ploufragan-Niort Laboratory and the Faculty of Veterinary Medicine at Utrecht University (Netherlands).
What was the study process ?
PB : We studied two groups of chickens, one vaccinated against IBV, and the other not. Three weeks after vaccination, we infected five animals in each group. After five days, we used the viruses produced by these animals to infect five more, then we repeated the operation again with five more animals. The aim was to mimic transmission of the virus between animals over time. At each stage, we analysed the genomes of the viruses produced.
What results did you get ?
PB : We found that the virus evolved rapidly in both groups of chickens. Its genome mutated compared to that of the virus originally used to infect the chickens. At the end of the infection cycle, the majority genome differed between the two groups : in the viruses from the vaccinated group, only one mutation had been selected compared to eight in the non-vaccinated group.
You might expect that the higher the number of mutations, the greater the impact on the virus, but this is not necessarily the case. Indeed, we know that a single, well-placed mutation can have greater consequences than several changes at locations that are less strategic, i.e. less decisive for the function of the mutated gene. The impact of each change therefore needs to be studied specifically.
What are the consequences for vaccination in livestock ?
PB : It is essential to vaccinate poultry to control infectious bronchitis. However, we saw in this study that the virus continued to evolve in the presence of vaccine-induced immunity. It is therefore really important to establish vaccine strategies that are capable, as far as possible, of stopping the viral infection completely.
To achieve this, it is important to choose the right vaccine, tailored to the virus strain that is circulating in the field. The closer the vaccine strain is to the one in circulation, the more effective the vaccine is.
Are these results transferable to the vaccination of humans against COVID-19 ?
PB : Not everything is comparable, they are not the same species and there are different issues facing humans and poultry. The vaccine types, mostly mRNA in humans and live attenuated vaccines in poultry, are very different. Nevertheless, it does help us anticipate the problems we might encounter in humans.
We have also begun a collaboration with Lyon University Hospital on the difference in the evolution of SARS-CoV-2 in vaccinated and non-vaccinated people, in order to compare the results in humans with those obtained in chickens.