Porcine reproductive and respiratory syndrome virus (PRRVS) is a major swine viral pathogen that affects the pig industry worldwide. Control of early PRRSV infection is essential, and different types of PRRSV-positive samples can reflect the time point of PRRSV infection. This study aims to investigate the epidemiological characteristics of PRRSV in China from Q4 2021 to Q4 2022, which will be beneficial for porcine reproductive and respiratory syndrome virus (PRRSV)control in the swine production industry in the future. A total of 7518 samples (of processing fluid, weaning serum, and oral fluid) were collected from 100 intensive pig farms in 21 provinces, which covered all five pig production regions in China, on a quarterly basis starting from the fourth quarter of 2021 and ending on the fourth quarter of 2022. Independent of sample type, 32.1% (2416/7518) of the total samples were PCR-positive for PRRSV, including 73.6% (1780/2416) samples that were positive for wild PRRSV, and the remaining were positive for PRRSV vaccine strains. On the basis of the time of infection, 58.9% suckling piglets (processing fluid) and 30.8% weaning piglets (weaning serum) showed PRRSV infection at an early stage (approximately 90% of the farms). The sequencing analysis results indicate a wide range of diverse PRRSV wild strains in China, with lineage 1 as the dominant strain. Our study clearly demonstrates the prevalence, infection stage, and diversity of PRRSV in China. This study provides useful data for the epidemiological understanding of PRRSV, which can contribute to the strategic and systematic prevention and control of PRRSV in China.

Porcine reproductive and respiratory syndrome virus (PRRSV) modified live virus (MLV) vaccination is used to control PRRSV. In China, farms conduct random sampling from sow herds every 4 to 6 months. They use the enzyme-linked immunosorbent assay (ELISA) method to monitor the immune status of the herd by tracking the positive rate or the sample-to-positive ratio. However, in farms that implement mass vaccination and have stable production, the positive rate of ELISA antibodies has decreased, especially in high-parity sows. This poses a considerable challenge to the current monitoring approach of PRRSV immunity. It remains unclear whether this reflects insufficient sensitivity of the kits for these special scenarios or the fact that the sows have truly lost immunity. In this study, 233 samples from four farms (A–D) across different regions of China were acquired. They were tested using four representative ELISA kits, two targeting the nucleocapsid protein (N) and two targeting the glycoprotein (GP) to evaluate PRRS immune status. The respective sample positive rates in A–D were 57.1–100%, 50.9–100%, 50–100%, and 75.7–100% using the kits. The positive rates using the four ELISA kits were 50.0–75.7%, 70.0–75.7%, 82.5–97.1%, and 100%, respectively, with poor agreement among them. The positive rates and humoral antibody levels for parity 1 and 2 sows were significantly lower than those with higher parities (>4). Eighty-eight ELISA-negative samples identified using ELISA kit A were verified using a viral neutralizing test (VNT), with only 15.9% of the samples testing negative. In conclusion, the ELISA antibody negativity issue existed, mostly occurring in specific farms tested using a specific kit. However, the low correlation with the VNT results and the poor agreements among the kits suggest that relying on one ELISA test is insufficient to monitor the immune status of PRRSV MLV-vaccinated herds.