Porcine reproductive and respiratory syndrome is estimated to cost the U.S. pork industry about $600 million per year. Thus, it’s easy to see that developing economical and effective PRRS-control strategies, including vaccines, remains an important goal for the pork industry.

On June 1-2, 2007, a meeting took place at the University of Illinois College of Veterinary Medicine to evaluate and discuss the state of current knowledge about PRRS vaccination. The meeting involved invited experts in the PRRS field, including virology, immunology and vaccinology. Attendees included clinical veterinarians, academics and vaccine industry scientists.

Three general questions were posed to the group. These included:

1.) How well do the current PRRS vaccines work?

2.) What do we need to know to improve PRRS vaccines?

3.) How likely is it that successful PRRS vaccines can be developed?

Most of the attendees agreed that current modified-live PRRS vaccines confer solid protection against some, but not all, PRRS viruses circulating in today’s production settings. The reasons for lack of protection against some PRRS viruses are unknown, but this is an area of active research. The attendees’ overall opinion was that killed-virus PRRS vaccines are ineffective or of limited efficacy at best, though some thought that they may be improved. The practice of serum inoculation, in which PRRS-naïve gilts entering the breeding herd are purposely infected with virus strains that are currently circulating on the farm, also has shown some efficacy. However, this presents some additional biosecurity risks. Current vaccine practices appear to be efficacious against homologous PRRS challenge, which provides support for vaccination as a viable PRRS-control strategy.

Improved PRRS vaccines would have properties that include rapid induction of immunity, protection against most currently prevalent PRRS virus strains, no adverse outcomes to swine health and the ability to differentiate vaccinated animals from infected ones. Administration simplicity to ensure compliance within production units is, of course, essential.

These properties define the goals for PRRS vaccine research and development.

So, what else is needed?

Given the belief that effective vaccination against PRRS is possible, what questions still need to be answered before improved vaccines are developed?

Clearly, we need to know more about the nature of the immune response to the PRRS virus. We also need to know what virus structures are important to protecting pigs and how these vary among PRRS viruses. Since PRRS virus is capable of impeding the pig’s immune response, understanding how this occurs and how to stop it are equally important.

In addition, to prevent adverse health outcomes we need to understand how PRRS virus causes disease so that safe and stable vaccine candidates may be rationally developed. Finally, we need to know how the production environment, overall pig health status, nutrition programs and other factors influence PRRS vaccination.

Most meeting attendees agreed that successful vaccination against PRRS virus can be achieved and improved — with current MLV vaccines as the standard by which improvement is defined. There was considerable discussion about the various types of vaccines, their likelihood of success and issues related to field efficacy. While there was some consensus that replicating vaccines showed the most promise, at this point all options remain open. Achieving efficacy, safety and logistical goals would be any vaccine’s primary criteria for success.

Once a candidate vaccine is identified, the development, regulatory and production process is predicted to take two to three years for standard modified-live and subunit vaccines, and four to six years for genetically modified-live vaccines. Attendees felt a realistic estimate is that improved vaccines will be in the hands of producers within five to 10 years.

While vaccination remains the control strategy of choice for PRRS, current vaccines are not adequate to prevent significant production and financial losses. Progress has been made toward providing the key scientific knowledge about PRRS that is required to produce improved PRRS vaccines, but important questions still remain unanswered.

In spite of these issues, there is considerable optimism that improved PRRS vaccines will be available to producers in the relatively near future.