The present invention relates to Infectious bronchitis virus (IBV) spike protein or a fragment thereof comprising a receptor-binding domain, wherein said spike protein or said fragment thereof is C-terminally fused to a trimerisation or tetramerisation domain. Such proteins form trimer or tetramer structures and as such they mimic the natural context of the spike protein. The present invention further relates to subunit vaccines comprising such a protein, to DNA molecules encoding such proteins, to plasmids comprising such DNA molecules, to avian live recombinant carrier viruses (LRCV's) comprising such DNA molecules or plasmids, to vaccines comprising such DNA molecules, plasmids and LRCV's and to combination vaccines comprising an IBV spike protein, DNA molecules, plasmids or LRCV's encoding such a protein, and another IBV vaccine capable of inducing protection against another IBV serotype and/or a vaccine capable of inducing protection against another avian pathogen.