Managing the Risks of Modified Live PRRS Vaccines in Swine Herds

Modified live virus (MLV) vaccines have played a major role in reducing the impact of porcine reproductive and respiratory syndrome (PRRS) by improving clinical outcomes and limiting virus circulation. Their ability to induce both humoral and cellular immune responses has made them the most widely used vaccine type for PRRS control worldwide^1,2,3,4. However, because these vaccines contain live attenuated virus capable of replication, their use also presents important safety considerations. Understanding these risks is essential for developing vaccination programs that maximize herd protection while minimizing unintended consequences. 

Vaccine Virus Transmission and Persistence 

Unlike killed vaccines, MLV vaccines replicate in vaccinated pigs, contributing to the development of protective immunity. However, this replication also creates the possibility of vaccine virus transmission. 

Evidence has shown that MLV viruses can spread from vaccinated to non-vaccinated pigs through direct contact. Vaccine virus has also been detected in oral fluids and semen, indicating that transmission may occur within breeding and production systems^2,5,6. These findings highlight the importance of considering herd structure, animal movement, and breeding management when implementing vaccination programs. 

Although transmission does not occur under every circumstance, the ability of vaccine viruses to circulate within susceptible populations should be considered during vaccination planning, particularly in herds with mixed immune status. 

Genetic Recombination: An Important Field Consideration 

PRRSV possesses a high mutation rate, and genetic recombination is recognized as one of the major mechanisms driving viral evolution. Because MLV vaccine viruses can replicate after administration, recombination between vaccine strains and circulating field strains may occur under field conditions^2,7,8. 

Several recombinant PRRSV strains containing genomic regions derived from vaccine viruses have been identified, demonstrating that vaccine-derived recombination can contribute to the emergence of genetically distinct viruses². The likelihood of recombination may increase when different MLV vaccine strains are used within the same production system or when vaccine viruses circulate alongside field strains. 

These observations reinforce the importance of selecting vaccination strategies carefully and maintaining ongoing surveillance of circulating PRRSV strains. 

Reversion to Virulence and Reproductive Risks 

Another recognized safety concern is the potential for reversion to virulence, in which an attenuated vaccine strain regains increased pathogenicity after replication. Although uncommon, this possibility has been documented and remains an important consideration when evaluating MLV vaccine use². 

Special attention is also required when vaccinating breeding animals. Vaccine virus has been shown to cross the placenta under certain conditions, and transplacental infection may occur when susceptible pregnant sows are exposed during late gestation. In addition, detection of vaccine virus in semen highlights the need for careful management of breeding boars and artificial insemination programs². 

These factors emphasize that vaccination protocols should be developed with consideration for the reproductive status of the herd and the potential consequences of vaccine virus dissemination. 

Balancing Vaccine Benefits with Safe Use 

Despite these safety considerations, MLV vaccines remain an essential tool for PRRS control because of their superior protective efficacy compared with killed virus vaccines^1,2,3. Their benefits in reducing clinical disease, viremia, and virus shedding continue to support their widespread use in swine production. 

Successful vaccination depends on balancing these benefits with appropriate risk management. Careful vaccine selection, consideration of herd health status, strategic vaccination timing, and continuous monitoring of circulating PRRSV strains all contribute to responsible vaccine use. 

Rather than limiting the value of MLV vaccines, understanding their biological characteristics enables veterinarians to make informed decisions that optimize protection while reducing the likelihood of unintended outcomes associated with vaccine virus circulation. 

Key Takeaway 

Modified live PRRS vaccines remain an important component of herd health programs, but their use requires careful consideration of vaccine virus transmission, recombination, reversion to virulence, and reproductive safety. Well-planned vaccination strategies and ongoing surveillance help maximize vaccine benefits while supporting effective long-term PRRS control. 

References 

1. Proctor J, Wolf I, Brodsky D, Cortes LM, Frias-De-Diego A, Almond GW, Crisci E, Negrão Watanabe TT, Hammer JM, Käser T. Heterologous vaccine immunogenicity, efficacy, and immune correlates of protection of a modified-live virus porcine reproductive and respiratory syndrome virus vaccine. Frontiers in microbiology. 2022 Sep 23;13:977796. https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2022.977796/pdf 

2. Li J, Miller LC, Sang Y. Current status of vaccines for porcine reproductive and respiratory syndrome: interferon response, immunological overview, and future prospects. Vaccines. 2024 Jun 1;12(6):606. https://www.mdpi.com/2076-393X/12/6/606 

3. Savard C, Alvarez F, Provost C, Chorfi Y, D’Allaire S, Benoit-Biancamano MO, Gagnon CA. Efficacy of Fostera PRRS modified live virus vaccine against a Canadian heterologous virulent field strain of porcine reproductive and respiratory syndrome virus. Canadian Journal of Veterinary Research. 2016 Jan 1;80(1):1-1. https://pmc.ncbi.nlm.nih.gov/articles/PMC4686029/pdf/cjvr_01_1.pdf 

4. Sirisereewan C, Woonwong Y, Arunorat J, Kedkovid R, Nedumpun T, Kesdangsakonwut S, Suradhat S, Thanawongnuwech R, Teankum K. Efficacy of a type 2 PRRSV modified live vaccine (PrimePac™ PRRS) against a Thai HP-PRRSV challenge. Tropical animal health and production. 2018 Oct;50(7):1509-18. https://www.academia.edu/download/85660272/s11250-018-1589-420220507-1-ejbrf3.pdf 

5. Yuan F, Sharma J, Nanjappa SG, Gaulke CA, Fang Y. Effect of killed PRRSV vaccine on gut microbiota diversity in pigs. Viruses. 2022 May 18;14(5):1081. https://www.mdpi.com/1999-4915/14/5/1081 

6. Foerster T, Streck AF, Speck S, Selbitz HJ, Lindner T, Truyen U. An inactivated whole-virus porcine parvovirus vaccine protects pigs against disease but does not prevent virus shedding even after homologous virus challenge. Journal of General Virology. 2016 Jun;97(6):1408-13. https://scholar.archive.org/work/5qocs6g4mzfldmefihswhw7swe/access/wayback / http://pdfs.semanticscholar.org/5611/c88442f45c3a50822c292c8707567aecddfc.pdf 

7. Fraile L, Sibila M, Nofrarías M, López-Jimenez R, Huerta E, Llorens A, López-Soria S, Pérez D, Segalés J. Effect of sow and piglet porcine circovirus type 2 (PCV2) vaccination on piglet mortality, viraemia, antibody titre and production parameters. Veterinary Microbiology. 2012 Dec 28;161(1-2):229-34. https://www.academia.edu/download/47175164/j.vetmic.2012.07.02120160711-8795-1rb04ki.pdf 

8. Yang S, Oh T, Mago J, Iwakuma A, Chae C. Optimal vaccination strategy against Mycoplasma hyopneumoniae, porcine reproductive and respiratory syndrome virus, and porcine circovirus type 2 in case of early M. hyopneumoniae infection. Veterinary Medicine and Science. 2020 Nov;6(4):860-74. https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/vms3.284