Types of vaccines against PRRS
At present, there are over 30 vaccines, although some of them are only available in their country of origin. Virtually, all these vaccines are based on live virus (modified live virus, MLV) or inactivated virus (IV), both on PRRSV1 and PRRSV2 -there are only two exceptions based on PRRSV2 subunits-, being MLVs the predominating vaccines used in the field nowadays.
So… are the current PRRS virus vaccines useful?
Although several vaccines are available worldwide, we know that they do not provide full and universal protection against PRRS virus infection. As stated above, there are two main reasons behind this problem: • The virus has high genetic diversity. • The virus strongly modulates the immune responses.
Can we predict protection between strains?
Unfortunately, at present the answer is absolutely negative. It is not possible to predict the level of protection that a given field strain or vaccine will afford against an infection with a new strain. As it has been discussed, the virus variability is extreme in many aspects (see section What is the PRRS virus?).
Immunisation of gilts and sows
Immunisation of gilts: Two of the most frequent methods to immunise gilts are Inoculation/contact of live-resident virus. Immunisation with live-resident virus is usually performed by using contact to presumably infected material, such as feces or animals, or by sera inoculation from a viremic piglet. Both strategies are potentially dangerous and the outcome is unpredictable.
Immunisation of piglets
The use of commercial vaccines should be the only possible option to immunise piglets. To vaccinate piglets, two important points must be considered in order to choose the best moment: • The development of immunity will require at least 3 weeks. • Maternal antibodies can interfere with the vaccines.
Routes of administration: Intramuscular versus intradermal
Currently all commercial vaccines are indicated for intramuscular administration. However, few of them are also indicated to inject by intradermal route. Compared to conventional intramuscular administration, intradermal injection in pigs are usually carried out using needle-free jet injectors. Needle-free administration in dermis offers important practical and immunity advantages over conventional intramuscular administration by methods based on needles and syringes:
What is the role of NAs and cell-mediated immunity?
Viral clearance after infection: Passive transfer studies have demonstrated that specific antibodies against PRRS virus are capable of protecting animals from homologous challenge. However, the role of NAs in the course of infection is a controversial issue. While some authors have reported a certain correlation between the appearance of NAs and the cease of viraemia or the clearance of the virus from lungs, other studies have suggested that for some strains, clearance of viraemia can occur without NAs; even more, viraemia and viral replication can persist even in the presence of NAs. In relation to cell-mediated immunity, some studies have reported that IFN-γ may be associated with viral clearance in the absence of NAs.