In general, PRRSV2 strains (formerly North American type or genotype II) are more virulent than subtype 1 PRRSV1, that is, the formerly designated as genotype I isolates circulating in Western Europe.
However, PRRSV1 isolates of other subtypes (2, 3 or 4) circulating in Eastern Europe can be of increased virulence.
PRRSv virulent strains
Additionally, some subtype 1 PRRSV1 isolates circulating in Italy or Belgium have shown the features of increased virulence.
Moreover, while reproductive pathogenicity is more or less similar for all isolates (with some differences though), PRRSV2 isolates seem to be more pneumotropic than PRRSV1 (with exceptions again).
So, if a scale of virulence was to be elaborated, the highly virulent Asian (Chinese) PRRSV2 isolates emerged in 2006 will be on top, followed by the highly pathogenic subtype 2 and 3 PRRSV1 of Eastern Europe, then the moderate virulence isolates of PRRSV2 and PRRSV1 followed by the less virulent isolates of any genotype.
The issue of heterologous protection is very different.
There are many evidences indicating that some level of heterologous (interspecies) protection exists although this is limited and, again, in general of lesser magnitude than the intraspecific protection.
Different papers indicated some beneficial effects of vaccination with PRRSV1 against PRRSV2 and vice versa (mainly improvement of zootechnical parameters and decrease of complications), while others indicated no effect.
The most likely scenario is that some level of protection exists but it is worse than in the homologous species. In any case, all heterologous protection (inter o intraspecific) has to be considered only partial.
If you want to learn more about protection between PRRSV strains, visit chapter 3 “Can we predict protection between strains?” of this site.
Centre for Research on Animal Health (CReSA), Barcelona University (UAB) – Spain