Evaluation of a strategy to enhance the efficacy and ease of application of porcine reproductive and respiratory syndrome virus vaccines.

Control of porcine reproductive and respiratory syndrome viruses (PRRSV) is sometimes hindered by the suboptimal immunogenicity and efficacy of existing vaccines. In addition, not following vaccine storage and administration conditions on-farm can negatively impact on the efficacy of current vaccines and present barriers to the development of more efficacious vaccination strategies. To address these challenges, we evaluated whether a 'prime and pull' immunisation strategy combining a prime with a 'needle-free' solid dose formulation of modified live PRRSV-1 vaccine (MLV), and a nasal boost with inactivated PRRSV-1 administered by an automated jet device can enhance protection against challenge infection. Forty-five, 4-week-old, PRRSV naïve, piglets were divided into 5 groups; including a benchmark group (10 pigs, vaccinated with a liquid MLV delivered by intramuscular injection), a prime only group (10 pigs vaccinated with the solid dose MLV), a prime/pull group (10 pigs vaccinated with the solid dose MLV and boosted 3 weeks later by nasal inoculation with inactivated MLV), an unvaccinated challenge control group (10 pigs) and a naïve group (5 pigs, without vaccination and PRRSV challenge). Pigs were challenged with a PRRSV-1 field strain 35 days post-vaccination and necropsied 10 days post-challenge. PRRSV viremia and nasal shedding were inferred by RT-qPCR, and antibody responses were assessed in serum and bronchoalveolar lavage by ELISA and virus neutralisation test. Whilst the nasal inactivated vaccine boost failed to augment efficacy, the solid dose vaccine provided neutralising antibody responses and protection (reduced viremia, virus shedding and gross lung lesions) comparable to the standard needle and syringe inoculation of PRRSV MLV suspension. These data support the further development of the solid dose formulation technology as a practical alternative to conventional delivery of PRRSV MLV and other vaccines to pigs.