This conference presentation by Vilalta, Sanhueza, Geary, Fioravante and Corzo gives an epidemiological overview of PRRS in filtered sow farm populations, synthesizing the field evidence available to 2018. Note that Leman is the name of the conference, not an author of the work.
By 2018 the underlying evidence base had reached a point where population level questions could be asked rather than efficacy questions. Dee and colleagues had reported in 2012 that odds of new PRRSV infection were 7.97 times higher before filtration than after across 38 herds, with median time to new infection of 30 months in filtered herds against 11 months in non filtered. Alonso and colleagues had reported in 2013 an approximately 80 percent reduction in risk of novel introduction across 20 filtered and 17 non filtered herds over seven years. Desrosiers and Cousin extended the population again in 2023, to 65 filtered farms of which 95.4 percent remained PRRSV negative.
The shift those numbers represent is from asking whether filtration works to asking what happens on the farms where it is installed. Filtered herds still experience breaks, at a lower frequency and over longer intervals, and the epidemiology of those remaining events is where the residual risk on a filtered site is described. For a producer already filtering, that is the more useful body of work, because the question is no longer whether to install a system but what else determines the outcome once one is running.