The present invention relates to nucleic acids comprising: (a) sequences of a replication competent transmissible gastroenteritis virus (TGEV), which sequences encode a TGEV replicase under the control of expression regulatory sequences, wherein the replicase is expressed in a host cell and will initiate replication of the nucleic acid and thus increase the number of nucleic acids in the cell； and (b) a sequence encoding at least one neutralizing epitope of ORF5 of porcine reproductive and respiratory syndrome virus (PRRSV), which sequence includes a disulfide bridge forming residue； and (c) a sequence encoding at least one further polypeptide capable of increasing an immune response against PRRSV. The present invention further relates to vectors, virus particles and host cells comprising these nucleic acids as well as their use for the preparation of vaccines, specifically for the preparation of vaccines.本發明係相關於一種核酸，包含：(a)一複製勝任傳染性胃腸炎病毒(TGEV)序列，該序列編碼一TGEV複製酶，在表現調控序列之控制下，其中該複製酶係於宿主細胞中表現，並會啟動核酸之複製，因而增加該核酸在該細胞中之數目；以及(b)一編碼豬繁殖與呼吸綜合症病毒(PRRSV)之ORF5之至少一中和性表位之序列，該序列包括一雙硫鍵形成殘基；以及(c)一序列，該序列係編碼至少一其他可增進對抗PRRSV之免疫反應之多胜肽。；本發明更相關於包含這些核酸之載、病毒顆粒與宿主細胞，以及其用於製備疫苗之用途，尤其是製備疫苗。
