PRRSV-2 infection chains in vaccinated and non-immunized pigs

This study examines the impact of modified live virus (MLV) vaccination on PRRSV-2 evolution using sequential pig-to-pig infection chains. Weaned pigs were divided into vaccinated (two vaccine strains) and non-immunized groups and challenged with PRRSV-2 sub-lineage 1A (1-7-4). Virus transmission continued for ~84 days across six batches, with serum samples from two days/batch with lowest Ct sequenced via the GridION platform (Oxford Nanopore Technologies, UK). Vaccinated pigs exhibited over twice the genetic divergence from the inoculum compared to controls. Deep sequencing revealed faster quasispecies shifts in vaccinated pigs, leading to more homogeneous populations. However, vaccinated pigs had significantly lower viral loads, suggesting fitness constraints on highly diverging variants. These findings demonstrate MLV vaccination drives PRRSV-2 evolution, emphasizing the need for ongoing genomic surveillance.