Surprisingly, the safety aspects of the modified live virus vaccines available today against PRRS have become a priority subject over more conventional issues such as their efficacy and cost-benefit. Considerable controversy has surrounded the question of whether these vaccines are safe to use under field conditions, because of the potential ability of the vaccinal viruses to mutate and become virulent.

More than 10 years ago a case of an American-type vaccine virus reverting to a virulent parent strain was confirmed after an in-depth molecular study, but the event had been observed only in one country where the vaccine was administered in very special conditions. The keys to developing a safe modified live virus (MLV) vaccine involve the virus attenuation process as well as selecting the right strain to be attenuated. Strain differences are important, however.

The European and American genotypes of the PRRS virus differ genetically (they share 60% of nucleotide identity) and also antigenically. Initially the American strains were thought to be the more likely to undergo frequent mutation, with the European strains tending to be more stable. However, studies reported in 2006 have demonstrated a high degree of variability also in European field strains of the virus. Nevertheless the European-like PRRS vaccine virus strains seem to be safer since no adverse effects of their variability have been reported so far, even though they have been used live in several countries worldwide.

An example from Europe

We can quote here the example of European-like strain VP-046 used in the commercial vaccine Amervac PRRS. Registration file data for the product show it has been both safe and efficacious under field conditions over the last 8-9 years. The same files at our company confirm there have been no reports of adverse effects after vaccinating several million pigs in Europe and Asia.

A trial requested by the European registration authorities showed that the strain remained highly stable and harmless despite being submitted to continuous replication among PRRS-naïve pigs. For the trial, piglets aged 4 weeks that had not been exposed to the PRRS virus were administered VP-046 at a rate 31.6 times bigger than in the conventional 2ml dose. It stayed stable after being passaged 4 times in the piglets and given one last passage in 90-day gestating sows (see Figure 1 ).

Stability is one issue relating to MLV vaccines, another is the possible shedding of the attenuated vaccine virus. In fact we should expect a short shedding period from pigs given an MLV vaccine. Some degree of vaccine virus shedding is likely, but in reduced numbers of pigs. Of course attenuated viruses must not induce clinical signs. But they should mimic (on a reduced scale) the replication of a wild virus. This replication is essential in order for the immune system to build up a solid protection, although the replicating capability of an MLV vaccine virus is smaller than that of field counterparts,

Every programme to prevent or eliminate PRRS would like to have a source of new replacement gilts that were protected without shedding the virus. Worth noting in this context is the success reported last year for a protocol of transitional vaccination within a more complex eradication programme. This Danish strategy used an MLV vaccine on naïve gilts that were afterwards confined in isolation for 12 weeks before their introduction into the sow herd.

Whole-herd vaccination

Mass vaccination protocols with MLV PRRS vaccines have sometimes been criticised for possibly compromising the vaccine's safety features. On the contrary, the evidence favours their use either at the start of a move to vaccinating regularly within an enterprise or before a PRRS virus eradication campaign. A report published in 2006 told how the virus was eliminated from 3 units by performing 2 mass vaccinations with a European MLV PRRS vaccine, combined with a herd closure period of 160-175 days before new replacement breeding animals were accepted. So proper MLV PRRS vaccine handling seems to give quite acceptable safety and efficacy results even in the most critical situations.

In the specific case of European MLV PRRS vaccines, a publication from 2005 refuted the possibility that implementing mass vaccination could induce a mutation or virus recombination. The study had undertaken a genetic analysis of 66 field strains of PRRS virus and of 3 European modified live vaccine viruses. The authors concluded that vaccination did not influence the evolution of field strains. Their view was reinforced by the fact that no differences in viral genetics were detected between countries that used vaccines and those that did not.

MLV vaccine viruses can also cross the placental barrier and reach the foetuses but without apparent disease effects. Recently, researchers inoculated either of 2 European vaccine virus strains into pregnant sows. Although both versions of virus crossed the placenta, neither the sows nor their subsequent progeny were affected adversely. This correlates with several field experiences where vaccine viruses have been protective and innocuous for pregnant sows and for their offspring.

Probably that is why the most effective first vaccination protocol currently, in sows which suffer a PRRS virus outbreak, vaccinates the whole breeding herd twice (regardless of stage of gestation) at an interval of a month. The approach is often proposed as a way of achieving a uniform and well-extended protection against field PRRS viruses.

It has happened in tests that pigs previously given an MLV PRRS vaccine also shed field virus after a challenge. But the shedding was reduced considerably, the actual extent depending on the degree of cross-protection, compared to non-vaccinated pigs. It would help to explain the observation that the infection pressure of a PRRS field virus is reduced after giving 2 mass vaccinations to the sows. In other words, the protection induced by the vaccine reduces the virus circulation in these sows as well as the severity of the clinical signs.

PRRS know-how today says an infection with a virulent strain does not guarantee at all the build-up of a proper, long-lasting immune protection. On the other hand, we have sufficient data to confirm that European MLV PRRS vaccines are able to induce an effective immune response when sows and pigs are vaccinated enough time ahead of a field-virus challenge. What is more, these vaccines can be considered safe.  PIGI