This review assembles the field record for air filtration against PRRSV across 65 filtered farms, of which 95.4 percent remained PRRSV negative. HEPA filtered boar studs in Quebec had remained negative since 2003. Farms using HEPA filtration showed 63 times lower incidence than farms using MERV 14 or MERV 16. The review also restates the approximately 80 percent reduction in the risk of novel PRRSV introduction reported by Alonso and colleagues.
As the most recent comprehensive review in this library, it extends the population beyond what the earlier studies covered. Alonso and colleagues in 2013 compared 20 filtered against 17 non filtered herds over seven years; Dee and colleagues in 2012 followed 38 herds and reported odds of new infection 7.97 times higher before filtration than after. The 80 percent figure this review attributes to Alonso belongs to the epidemiological paper in Preventive Veterinary Medicine 112, not to the financial companion published in volume 111, and the two are frequently conflated in secondary material.
The HEPA comparison invites care in how it is read. It describes incidence across farms using different filtration classes, and farms that install HEPA differ from farms that install MERV 14 in more than the filter, including what a boar stud or a nucleus site justifies spending and what other controls accompany the investment. The defensible reading is that higher efficiency media is associated with lower incidence in the field record, alongside the controlled result from Dee and colleagues in 2005 that MERV 14 prevented aerosol transmission under experimental conditions. Selecting a class for a particular building remains a question about that building's exposure, its airflow requirement and its pressure budget.