Comparison of porcine reproductive and respiratory syndrome modified live virus and inactivated virus vaccines against NADC34-like PRRS virus challenge in the field.

Porcine reproductive and respiratory syndrome (PRRS) is responsible for tremendous financial losses worldwide in the swine industry. Prevention and control of PRRS is still an enormous challenge for the swine industry. There is no unified view on how to minimise losses in infected herds. Here, we conducted a retrospective analysis after outbreaks in farms with modified live virus (MLV) and inactivated virus (KV) vaccination protocols in the field. This involved tracking and analysing production data and diagnostic test results from 11,091 sows over an 84-week period, including before, during, and after the PRRS virus (PRRSV) outbreak. Long-term observations revealed that the MLV-vaccinated farm returned to control line swine production metrics, including the return-to-oestrous rate (42 days), abortion rate, farrowing rate, mummified foetus rate, and live-born and preweaning mortality rates, than the KV-vaccinated farm did. However, the field strain positive rates of Farm B with the KV protocol remained relatively low (P < 0.05) during the beginning of first breeding cycle (0-3rd month), whereas those of Farm A with the MLV protocol increased (P < 0.01) during the first breeding cycle (0-4th month). Furthermore, the heterogeneity of the PRRSV antibody S/P values increased before 20 and 10 days before the PRRS outbreak, as determined by Kolmogorov‒Smirnov normality tests (P < 0.05). Our results revealed the characteristics of serum antibody changes preceding PRRS outbreaks and the impact of different types of vaccines on production metrics and PRRS field infection rates in piglets after outbreaks, providing a reference for subsequent sow management protocols before, during, and after PRRS outbreaks.