Serological prevalence of PCV2 in various farm sizes. The farms were categorized into 3 sizes, including small (50-500 sows), medium (501-5,000 sows) and large (>5,000 sows) sizes. The immune response levels are presented in three categories, negative (S/P ratio of 0.49 or less or the titer of ≤1,070), low positive (S/P ratio of 0.5-2.3 or the titer of 1,071-5,766) and high positive (S/P ratio of 2.31 or greater or the titer of ≥5,767).