Infectious bronchitis virus (IBV), which causes respiratory disease in chickens, has spikes on its surface which are used to attach to the respiratory cells of chickens.

The spike is made of two proteins call S1 and S2. S1 is the primary protein that the chicken's immune system recognises and to which it produces an immune response. Small changes in the amino acid sequence of the S1 protein can make major differences in how the spike binds to chicken cells and how the chicken's immune system recognises the virus.

However, the make-up of the S1 protein is notoriously prone to variation, which can make vaccines less effective when they no longer match the virus.

The Arkansas (Ark) live IBV vaccines contain a virus isolate called ArkDPI. It has been shown that this vaccine contains virus subpopulations that vary in the amino acid sequence of the S1 protein, which can lead to reduced protection from field strains of the virus.

Dr Brian Jordan and Dr Mark Jackwood looked into the effect of the two most common changes in the amino acid sequence of the S1 protein found in the virus subpopulations in the ArkDPI vaccine.

One of the changes increased the ability of the virus to bind to respiratory cells, whilst the other decreased this ability, but increased binding ability to chicken embryo cells. The virus with the second change also did not effectively induce antibodies against field IBV challenge.

Since the vaccine is produced in chicken embryos, these characteristics could increase the concentration of the virus with the second amino acid change in the vaccine population, leading to a less effective vaccine.

The report from funders USPOULTRY and the USPOULTRY Foundation said that this understanding provides the basis for development of an improved vaccine, and shows how important small changes in vaccines can be for chicken protection.