Current PRRS Vaccines: Understanding Killed and Modified Live Vaccines

Vaccination remains a key component of porcine reproductive and respiratory syndrome (PRRS) control, helping reduce disease severity and improve herd health. However, the extensive genetic diversity of PRRSV continues to challenge vaccine performance, particularly against heterologous strains^1,2,3. Currently, modified live virus (MLV) and killed virus (KV) vaccines are the two principal vaccine types available for PRRS control. While MLV vaccines are widely used because of their superior immunogenicity, KV vaccines continue to offer advantages in terms of safety, creating an important balance between efficacy and risk in vaccination programs. 

Understanding the strengths and limitations of both vaccine types is essential for selecting vaccination strategies that align with herd health objectives and disease pressure. 

Modified Live Vaccines: The Mainstay of PRRS Control 

Modified live vaccines remain the most widely used PRRS vaccines worldwide because of their ability to provide protection against respiratory disease and induce both humoral and cellular immune responses^3,4,5,6. 

Evidence indicates that PRRSV-2 MLV (MLV2) vaccines are effective in reducing respiratory disease and can provide varying levels of protection against genetically diverse PRRSV-2 strains³. Despite more than two decades of field use, MLV vaccines continue to demonstrate protective efficacy against recently identified recombinant PRRSV-2 strains in different regions^7,8. 

Vaccination of pregnant sows has also been shown to enhance maternally derived neutralizing antibodies in piglets. These piglets experience delayed and comparatively lower viral loads following exposure to wild PRRSV infection, contributing to improved survival during the early postnatal period. Elevated neutralizing antibody levels have been maintained for at least four weeks after birth⁹. 

Although MLV vaccines generally do not induce sterilizing immunity against heterologous viruses, they frequently provide partial cross-protection. This protection is reflected by delayed onset of viremia, fewer days of clinical fever, reduced lung lesions, and milder clinical signs compared with unvaccinated animals⁴. In addition, MLV2 vaccines have demonstrated protection against PRRSV-1 challenge, reducing viremia, lung lesions, and PRRSV-positive cells through the induction of IFN-γ-secreting immune cells^2,3,4. 

Challenges Associated with MLV Vaccines 

Despite their advantages, vaccine efficacy is influenced by the genetic relationship between vaccine strains and circulating field viruses. Protection against heterologous PRRSV strains remains incomplete, and vaccine performance may vary depending on the challenge strain^1,2,3. 

In regions where both PRRSV-1 and PRRSV-2 circulate simultaneously, no commercially available MLV vaccine currently provides complete protection against both viral species. Under these circumstances, the use of a single MLV2 vaccine has emerged as a practical clinical approach for controlling both PRRSV types⁴. 

Recent advances have also demonstrated the potential of chimeric vaccines targeting genetically diverse PRRSV-2 strains. Vaccinated pregnant sows showed no significant post-vaccination viremia, while their piglets exhibited reduced viral loads and fewer lung lesions compared with piglets from unvaccinated sows, highlighting the promise of broader vaccine protection in future vaccination strategies¹⁰. 

Killed Virus Vaccines: Prioritizing Safety 

Killed virus vaccines offer an excellent safety profile because they cannot replicate, mutate, or spread within vaccinated animals⁴. These vaccines are licensed in several European countries and other parts of the world, although their use remains limited compared with MLV vaccines. 

Their primary limitation is reduced immunogenicity. Even after repeated immunization, KV vaccines generally induce weaker humoral immune responses and provide limited protection against field PRRSV infections, particularly those caused by heterologous strains⁴. Likewise, using KV vaccines as booster doses following MLV vaccination has not consistently improved protection against clinical disease, virus shedding, or gross lesions¹¹. 

Nevertheless, KV vaccines continue to demonstrate value in specific situations. Autogenous inactivated vaccines administered to pregnant sows have been associated with partial protection of piglets against PRRSV-2, resulting in reduced lung pathology and improved weight gain through the transfer of maternal neutralizing antibodies¹². 

Key Takeaway 

Modified live vaccines remain the primary option for PRRS control because they provide stronger immune responses and broader protection than killed vaccines, although protection against genetically diverse field strains remains incomplete. Killed virus vaccines offer superior safety but generally provide more limited protection, highlighting the need to balance efficacy and safety when designing herd-specific vaccination programs. 

References 

1. Madapong A, Saeng-Chuto K, Boonsoongnern A, Tantituvanont A, Nilubol D. Cell-mediated immune response and protective efficacy of porcine reproductive and respiratory syndrome virus modified-live vaccines against co-challenge with PRRSV-1 and PRRSV-2. Scientific reports. 2020 Feb 3;10(1):1649. https://www.nature.com/articles/s41598-020-58626-y.pdf 

2. Oh T, Kim H, Park KH, Yang S, Jeong J, Kim S, Kang I, Park SJ, Chae C. Comparison of 4 commercial modified-live porcine reproductive and respiratory syndrome virus (PRRSV) vaccines against heterologous Korean PRRSV-1 and PRRSV-2 challenge. Canadian Journal of Veterinary Research. 2019 Jan 1;83(1):57-67. https://pmc.ncbi.nlm.nih.gov/articles/PMC6318830/pdf/cjvr_01_57.pdf 

3. 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 

4. 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 

5. 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 

6. 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 

7. Jeong J, Choi K, Kang I, Park C, Chae C. Evaluation of a 20 year old porcine reproductive and respiratory syndrome (PRRS) modified live vaccine (Ingelvac® PRRS MLV) against two recent type 2 PRRS virus isolates in South Korea. Veterinary Microbiology. 2016 Aug 30;192:102-9. http://dx.doi.org/10.1016/j.vetmic.2016.07.006 

8. Wei C, Dai A, Fan J, Li Y, Chen A, Zhou X, Luo M, Yang X, Liu J. Efficacy of Type 2 PRRSV vaccine against challenge with the Chinese lineage 1 (NADC30-like) PRRSVs in pigs. Scientific Reports. 2019 Jul 25;9(1):10781. https://www.nature.com/articles/s41598-019-47239-9.pdf 

9. Hsueh FC, Wang SY, Lin WH, Lin CF, Tsai CY, Huang CW, Sun N, Chiou MT, Lin CN. Correlation of neutralizing antibodies (NAbs) between sows and piglets and evaluation of protectability associated with maternally derived NAbs in pigs against circulating porcine reproductive and respiratory syndrome virus (PRRSV) under field conditions. Vaccines. 2021 Apr 21;9(5):414. https://www.mdpi.com/2076-393X/9/5/414 

10. Jeong CG, Khatun A, Nazki S, Kim SC, Noh YH, Kang SC, Lee DU, Yang MS, Shabir N, Yoon IJ, Kim B. Evaluation of the cross-protective efficacy of a chimeric PRRSV vaccine against two genetically diverse PRRSV2 field strains in a reproductive model. Vaccines. 2021 Oct 31;9(11):1258. https://www.mdpi.com/2076-393X/9/11/1258 

11. Li C, Liu Z, Chen K, Qian J, Hu Y, Fang S, Sun Z, Zhang C, Huang L, Zhang J, Huang N. Efficacy of the synergy between live-attenuated and inactivated PRRSV vaccines against a NADC30-like strain of porcine reproductive and respiratory syndrome virus in 4-week piglets. Frontiers in Veterinary Science. 2022 Feb 2;9:812040. https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2022.812040/pdf 

12. Kick AR, Wolfe ZC, Amaral AF, Cortes LM, Almond GW, Crisci E, Gauger PC, Pittman J, Käser T. Maternal autogenous inactivated virus vaccination boosts immunity to PRRSV in piglets. Vaccines. 2021 Jan 31;9(2):106. https://www.mdpi.com/2076-393X/9/2/106