Viral subpopulation variability in different batches of Infectious bronchitis virus (IBV) vaccines based on GI-23 lineage: Implications for the field

The control of infectious bronchitis (IB) is largely based on routine vaccine administration, often using live-attenuated vaccines. However, their capability to replicate and be transmitted among animals and farms im-plies significant risks. The detection of strains genetically related to vaccines complicates the diagnostic process and understanding of the viral molecular epidemiology. Moreover, reversion to virulence and associated clinical outbreaks can occur although the underlying mechanism are often unknown. In the present study, three vaccine vials, based on IBV GI-23 lineage (also known as Variant2) were deep sequenced through Next Generation Sequencing (NGS) to investigate the presence and features of viral subpopulations. To elucidate the consequences in the field and identify potential markers suitable for a DIVA strategy, the S1 sequences of strains originating from farms in different countries were sequenced and classified based on the knowledge of their vaccination history and similarity with the applied vaccine. Although all considered vaccine batches shared the same consensus sequence, different subpopulations were identified suggesting independent and poorly constrained evolutionary processes. When compared with strains sampled from farms, the vaccine consensus sequences and the respective subpopulations clustered with vaccine strains and no genetic features were consistently shared with field strains. Therefore, if vaccine-induced outbreaks occur, they are more likely to originate from in vivo evolution rather than selection of already present subpopulations. Although some amino acid residues were most commonly detected in field or vaccine strains, no consistent marker could be identified.The occurrence of subpopulations within IBV GI-23-based vaccines and variability featuring different pro-duction batches was demonstrated. Being such a phenomenon apparently driven by random genetic drift rather than directional selection, the differentiation between field and vaccine-derived strains appears extremely challenging based on sequence analysis alone. The knowledge of farm management and vaccination history should thus be considered for a proper epidemiological investigation.