A swine vet consultant recommends not to use a PRRS live vaccine because the efficacy of the vaccine is not enough to control the field viruses. If there is an outbreak of PRRS with clinical symptoms, how long will it take to be a stable situation without vaccination? Or should we use a vaccine to make and keep a PRRS stable farm?
PRRSV MLVs are not able to completely control infections, particularly not with heterologous field virus, so no sterile immunity is induced by the use of MLVs. However, hardly any vaccine will induce sterile immunity and prevent infection.
The main goal of vaccines is to control clinical disease and reduce economic losses. These effects can also be achieved by PRRSV MLVs; those vaccines are able to reduce clinical signs both in sows/gilt and piglets, so they are able to reduce reproductive disorders and respiratory signs and improve growth performance.
It could be demonstrated in the past that in addition to the use of MLVs, additional measures are necessary to reduce viral load in affected farms in order to control PRRS; experimental data has shown that vaccinated piglets are protected by field virus infection up to a certain infectious dose; if the infectious dose (i.e. the viral pressure in farms) is too high, MLVs are not delivering satisfying results. Therefore, internal biosecurity measures reducing internal virus circulation (e.g. McREBEL measures) are crucial in the control of PRRS.
After a clinical outbreak, based on our experience clinical signs will be present on affected farms for a few months; for sure the situation depends on the virus strain involved; in general, I would assume that sows which receive vaccination in mid gestation will be the first one to deliver normal litters; all sows in later stages of gestation during the acute outbreak can be assumed to be affected by PRRSV, no matter if vaccinated or not.
The question if a vaccination is recommended or not in case of an outbreak is for sure a good one and can be discussed controversially; on the one hand it can be assumed that the sow herd will become immune, and all sows will get infected. The effect of the vaccine should be to harmonize the immune status in affected farms, speed up the process of herd immunization and to a certain degree also replace field virus by the MLV.
In my own herd I would use mass vaccination with MLV to be on the safe side.
If you want to know more about vaccination in pigs with PRRS MLV read out chapter: So… are the current PRRS virus vaccines useful?
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University of Veterinary Medicine, Vienna
