Published on: April 28, 2016
We should remember that the protection means different things for virologists and practitioners and producers. The best is when immunization protects completely against the infection. This is rarely achieved in the field conditions.What we are aiming at, using immunization protocols, is to protect against the clinical symptoms, and restricting virus shedding and transmission. In other words we are aiming at clinical protection and limiting the virus presence in the farm environment.
The cross protection is best if induced by the same strain that challenges the pigs. Some scientific reports show that it can be lifelong, not only in terms of clinical symptoms but also can protect against the infection. The further away antigenically is the challenging virus from the immunizing virus , the lower is the cross protection. The problem is that we don’t know how to measure antigenic relationship between strains that would allow us to predict the level of cross protection (also vaccine induced).
Today there are modified live virus vaccines produced from multiple genetic lineages of either Type 1 or Type 2. In theory better protection is provided by the vaccine of the same genotype as the wild type virus. Genetic classification of the vaccine strain within the genotype is less important. However, there are reports showing good efficacy of “American” vaccines against “European” strains, and vice versa. We have to remember that the vaccination efficacy is a result of the vaccine antigenicity , vaccination technique, the properties of the wild type strain and its dose (the presence in the environment). All these factors influence the outcome of the vaccination protocol. “Weak” vaccine could be good enough if properly administered in animals staying in clean pens and are fed well. “Strong” vaccine will not work if administered as split dose, in animals living in poor conditions, coinfected with other pathogens that may compromise their immune system, and in a population of mixed immune status regarding PRRSV. Large farms can be coinfected with multiple strains of PRRSV . It is good to know it, and to monitor the changes in virus populations during the control programs.
In summary, the differences in immunogenicity between the vaccines do exist but in the field conditions the most important is to be consistent in executing the vaccination program with a given vaccine, to improve management practices and to monitor the effects of the program in terms of changes in virus circulation (laboratory diagnosis) as well as in terms of production parameters.