Jiayu Liu ¹ Xinyu Yan ^2* Wu Wensi ¹ Yan Li ^1* Shuaibin Xing ^1* Shan Zhao ² Xiaobo Huang ² QIN ZHAO ² Yiping Wen ^2* Sanjie Cao ² Senyan Du ² Qigui YAN ^2* Nanfang Zeng ^1*

• ¹ Giantstar Farming & Husbandry Co., Ltd, Chengdu, China
• ² College of Veterinary Medicine, Sichuan Agricultural University, Yaan Shi, China

At present, the NADC30-like strain has become the prevalent strain of PRRSV in China. Many studies have found that existing commercial vaccines are ineffective or provide only limited protection. This study assessed the effectiveness of various dosages against a NADC30-like PRRSV infection using commercial PRRSV vaccines, Ingelvac PRRS MLV, which have been widely utilized in China. 35 days after vaccination, piglets had a nasal infection with a NADC30-like CF PRRSV.Compared to those piglets immunized with 1 dosage, the piglets immunized with 0.01 dosage had better performance, such as the highest average daily gain before the challenge, lesser lesions and viremia after the challenge, low clinical score, and stable temperature during the study. However, the piglets immunized with 0.01 dosage still showed viremia, viruses were detected in their lungs, tonsils, and inguinal lymph nodes, and pathological lesions occurred in their lung.Immunohistochemistry staining of the lung of vaccinated piglets revealed a similar viral load to that of unvaccinated piglets, suggesting that immunization could not completely remove the virus from the vaccinated piglets' tissues. Therefore, such observations suggested that the MLV vaccine could provide extremely limited protection against the NADC30-like PRRSV infection, and lowering the dosage to 0.01 may produce better protective efficacy.