PRRS MLV vaccination is a common and effective strategy used by the pig industry to mitigate PRRSV-attributed impact. However, there is little information on the impact of this intervention on the production performance. This study assess the impact of PRRS MLV mass vaccination of PRRSV-stable breeding herds on wean-to-feeder mortality using natural experiments under field conditions. Data from 331 growing pig groups, from PRRSs-stable farms and using MLV vaccination, was collected and classified into 5 groups, based on the period between the MLV intervention and the weaning date. Baseline groups were defined as groups weaned six weeks after the MLV intervention, and treatment groups were the batches weaned every 3 weeks after intervention. There was a significant increase of 1.31% of mortality on groups weaned 4 to 6 weeks after the MLV intervention, compared to baseline. The estimated impact of mass vaccinating breeding herds with PRRS MLV on pig growth performance was $1.52. Based on this value, the practice of vaccination of sow herds, with PRRS MLV was beneficial on herds that report outbreak with PRRSV every 3.7 years or more frequently. The results of this study provide information to help swine veterinarians make better informed decisions regarding the use of PRRSV MLV vaccine and other interventions in the sow herd to reduce growth mortality. To access the report with detailed findings please visit the AASV Foundation-funded Research page, or view the report directly.