The objective was to evaluate the effects of increasing the dietary g SID Lys:ME from 100% (NRC requirement) to 120% of the requirement of grower pigs, in vaccinated (vac+; MLV Ingelvac PRRS®) and non-vaccinated [vac-; no vaccination for Porcine Reproductive and Respiratory Syndrome (PRRS)] pigs that were then challenged with a PRRS virus. Secondly, we evaluated two diet formulation approach to achieve a 120% SID Lys:ME ratio by either increasing Lys relative to energy or diluting energy relative to Lys. Within vaccine status, 195 mixed-sex pigs vac+ (35.2 ± 0.60 kg BW) and vac- (35.2 ± 0.65 kg BW) were randomly allotted to one of three dietary treatments: The treatments (g SID Lys:ME) were 2.67 (100%, control), 3.23 (120%, increased SID Lys), and 3.22 (120S%, dilution of ME via dietary inclusion of sand). Pigs were randomly allotted, within vaccine status, to 24 pens (7-10 pigs/pen; n=8). All pigs were inoculated with a PRRS virus and started on experimental diets on d 0. Over a 42 d challenge period, BW, ADG, ADFI and G:F were determined and all data was analyzed within vaccination status. In vac+ pigs compared to control, end BW increased in 120% and 120S% fed pigs (10% and 9% respectively; P < 0.05) and ADG increased by 20% and 17% respectively (P < 0.05). Similarly, in vac- pigs a 9% and 20% improvement in end BW and ADG was observed respectively compared to control (P < 0.05). Overall, ADFI increased by 20% in the 120S% vac+ pigs (P = 0.003) and by 17% in vac- pigs (P = 0.001) compared to control. The 120% vac+ pigs had the greatest G:F compared to control and 120S% pigs (P < 0.01).