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PRRSV-1 modified-live virus vaccine cross-protects against three major PRRSV subtypes prevalent in Europe

New studies show that a commercial vaccine for porcine reproductive and respiratory syndrome virus (PRRSV) cross-protects against isolates from the three major subtypes of the virus found in Europe, reducing clinical symptoms, viremia and costly performance losses associated with infection.

The modified-live virus (MLV) vaccine is based on PRRSV-1 subtype 1, the most prevalent strain of the virus in Europe. Approved for use in 1-day-old pigs, the vaccine has been shown to induce a robust and long-lasting immune response, even in the presence of maternal antibodies.1,2

However, according to Marta Cabana, a research and development scientist at Zoetis, the vaccine’s ability to cross-protect against PRRSV-1 subtypes 2 and 3 — which are less common but more virulent3 — was previously assumed to be limited.

“The high level of genetic diversity among PRRSV strains from different subtypes made it reasonable to predict incomplete protection,” she explained. “This hypothesis was reinforced by available literature on other commercial PRRSV vaccines against different subtypes, mainly Eastern European variants, which showed limited cross-protective efficacy.”4

To test the efficacy of the MLV vaccine against PRRSV-1 subtypes 1, 2 and 3, scientists from Zoetis conducted three controlled challenge studies in Austria, UK and Spain, employing a study design that Cabana says is the “gold standard” for measuring cross-protection.1,5

“It’s difficult to evaluate cross-protection of PRRSV vaccines under field conditions because the outcome could be the result of a combination of different factors such as the virus variant, herd-management practices and concomitant pathogens,” she explained.

“By running studies under well-controlled, experimental conditions, we can extract conclusive efficacy data that is linked specifically to the strains we are inoculating against, and not masked by any confounding factors.”

Challenge studies

In each of the three studies, investigators divided 1-day-old PRRSV-negative piglets into two groups (20 to 29 pigs each), with one group receiving the vaccine and the other serving as negative control.6,7

Four weeks later, they challenged all pigs in each study with a virulent strain of one of the main PRRS-1 subtypes (1, 2 or 3). They then recorded clinical signs over the course of 10 days and measured bodyweight before challenge and before necropsy. They also measured PRRSV viremia and shedding (nasal and oral) and evaluated lung macroscopic lesions at necropsy.

Results from the three studies demonstrated a cross-protective effect of vaccination against all three PRRSV-1 subtypes:

Subtype 1 study: Unvaccinated piglets showed a higher incidence of depression and respiratory distress after challenge. Average daily weight gain (ADWG) was significantly higher in vaccinated pigs. Viremia and nasal and oral shedding were significantly reduced in vaccinated pigs at most of the monitored time points after challenge. The percentage of lung lesions in vaccinated pigs (2%) was significantly lower than in control pigs (6%).

Subtype 2 study: Unvaccinated piglets showed a higher incidence of depression and abnormal respiration after challenge. Bodyweight and ADWG were significantly higher (p < 0.05) in vaccinated pigs at necropsy. Viremia and nasal and oral shedding were significantly reduced (p < 0.05). The percentage of lung lesions in vaccinated pigs (3%) was significantly lower (p < 0.05) than in control pigs (15%).

Subtype 3 study: Unvaccinated piglets showed a higher incidence of coughing, depression and conjunctivitis after challenge and general condition was lower. ADWG was significantly higher in vaccinated pigs at necropsy. Viremia was significantly reduced (p < 0.05) in vaccinated pigs at days 5 and 10 after challenge. The percentage of lung lesions in vaccinated pigs (2.8%) was also lower than in control pigs (6.8%).

Based on these results, investigators concluded that giving pigs the new MLV vaccine at 1 day of age reduced infection, disease severity and performance losses following challenge with pathogenic strains of PRRSV-1 subtypes 1, 2 and 3.

Broad and early protection

According to Cabana, the findings confirm that MLV vaccines are valuable tools for protecting against the potentially devastating effects of PRRS, which cost European pig producers approximately €1.5 billion per year.8

“Currently, the geographic distribution of PRRSV-1 subtypes in Europe is clearly divided by Poland’s eastern border, with subtype 1 prevalent to the west and subtypes 2 and 3 more common to the east,” she said.

“However, with increased movement of pigs within Europe, it is reasonable to see more Eastern European strains in Western European regions, making cross-protection increasingly important.

“The broad, early and long-lasting protection provided by the MLV vaccine used in this study makes it an excellent management tool for protecting pigs from PRRSV-1 as well as costly secondary infections.”





1 Fort M, et al. Vaccination of 1-day-old pigs with a PRRSV modified-live attenuated vaccine confers protection until slaughter. ESPHM 2018 Abstract Book.
2 Balasch M, et al. Vaccination of 1-day-old pigs with a porcine reproductive and respiratory syndrome virus (PRRSV) modified live attenuated virus vaccine is able to overcome maternal immunity. Porcine Health Management. 2018:4:25.
3 Stadejek T, et al. Pathogenicity of three genetically diverse strains of PRRSV Type 1 in specific pathogen free pigs. Vet Microbiol. 2017;209:13-19.
4 Bonckaert C, et al. Modified-live PRRSV subtype 1 vaccine UNISTRAIN® PRRS provides a partial clinical and virological protection upon challenge with East European subtype 3 PRRSV strain Lena. Porcine Health Management 2016;2(1):12.
5 Cabana M, et al. Assessment of the efficacy of a PRRSV-1 subtype 1-based modified-live virus attenuated vaccine in front of the challenge with subtype 2 and subtype 3 PRRSV-1 strains. Data on file.
6 Ibid.
7 Kreutzmann H, et al. Efficacy of a novel vaccine against PRRSV in neonatal piglets. ESPHM 2019 Abstract Book.
8 De Paz X. PRRS cost for the European swine industry. Pig333.com. August 2015. Located at https://www.pig333.com/articles/prrs-cost-for-the-european-swine-industry_10069/. Accessed 14 May 2020.








Posted on September 8, 2020

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