Live attenuated vaccines are critical in the control of avian infectious bronchitis. It is necessary to know the protection conferred by commonly used commercial live vaccines. In this study, specific pathogen-free chicks were vaccinated with the commercial live vaccines H120, 4/91 and LDT3-A. Blood samples were collected at weekly intervals for the detection of IBV-specific antibodies and quantification of CD4+ and CD8+ T lymphocytes. At 21 days post-inoculation the vaccinated birds were challenged with the IBV prevalent local strains GX-YL5, GX-GL11079 and GX-NN09032, respectively. Trachea and kidney samples were collected at 5 days post-challenge for the detection of the virus. The results showed that the H120 group exhibited medium antibody levels, the lowest percentages of CD4+, CD8+ T lymphocytes and the highest viral loads. The 4/91 group showed the lowest antibody levels, but the highest percentages of CD4+, CD8+ T lymphocytes and the lowest viral loads. The LDT3-A group showed the highest antibody levels, the medium percentages of CD4+, CD8+ T lymphocytes and the medium viral loads. The protection rates of H120, 4/91 and LDT3-A groups were 41.7-58.3%, 75.0-83.7% and 66.7-75.0%, respectively. The present study demonstrated that the vaccines H120, 4/91 and LDT3-A could stimulate the immunized chicks to produce different levels of humoral and cellular immunity to resist the infection of IBV, but couldn't provide complete protection against the prevalent local strains of IBV in southern China. Also, the vaccine 4/91 offered the best immune protection among the three vaccines.
Keywords: commercial live vaccine; immune protection; infectious bronchitis virus; prevalent local strains; southern China.