PCV2 vaccines remain effective despite growing genetic diversity
The emergence of new genetic variants of porcine circovirus 2 (PCV2) has raised concerns about the ability of commercial PCV2 vaccines to protect pigs from infection. However, a recent publication in Pathogens confirms that current vaccines continue to be effective, suggesting the existence of a single PCV2 serotype despite the significant and growing number of genotypes.1
PCV2 vaccines have “remarkably decreased the impact of PCV disease and the detrimental effects of subclinical infection” since they first became available, noted authors Giovanni Franzo, of Italy’s University of Padua, and Joaquim Segalés, of Spain’s Universitat Autónoma de Barcelona and Institute of Agrifood Research and Technology. However, they added, PCV2 is characterized by a high mutation rate that has led to the emergence of many variants over time.
The impact of PCV2 genetic and phenotypic diversity on vaccine efficacy is therefore a topic of intense debate, particularly after the emergence of PCV2d. The vast majority of currently available vaccines are based on PCV2a or its capsid protein, they noted, and the discovery of different neutralizing epitopes among genotypes could justify the concern regarding differential cross-protection and the presence of vaccine-escape variants.
Multiple genotypes, one serotype
To shed light on this debate, Franzo and Segalés conducted a comprehensive review of available knowledge on PCV2 genetic heterogeneity, immunity and vaccine efficacy, organizing and harmonizing the available data from different sources. Although genotype variability is likely to increase in the future, the researchers found that cross-immunity is present among major genotypes (PCV2a, PCV2b and PCV2d).
“This ensures, thus far, the vaccine-induced protection by those products based on PCV2a. Therefore, to date, we consider that the different genotypes of PCV2 still represent one single viral serotype,” they wrote.
“In other words, PCV2 genotypes conform to a unique immunological variant with common antigenic properties covered by existing commercial vaccines.”
In all instances, PCV2 vaccines appeared to effectively reduce clinical signs, viremia, viral shedding and lesions while inducing adequate humoral and cellular immune responses, they noted.
“No obvious evidence of a clinically significant differential cross-protection among PCV2 strains could be proven, and all commercial vaccines seem effective in preventing the most severe outcomes of PCV2 infection.”
Additional valency may improve protection, limit viral evolution
Although proper herd management is critical, they wrote, the improvement and update of PCV2 vaccines could likely have further benefits for animal health and performance.
“The inclusion of additional valency in vaccines has been suggested by several authors and recently implemented in some commercial vaccines, where…recombinant chimeric PCV1 expressing the porcine circovirus type PCV2a and PCV2b ORF2 genes was developed,” the authors stated.
“The presence of PCV2b, in addition to the direct improved protection against homologous challenge, could be beneficial against other genotypes, both because of its closer genetic distance than PCV2a and because of a likely increase in protection breadth due to the presence of a combination of different epitopes.”
The beneficial effect of different epitope combinations rather than simple genetic resemblance is scientifically supported, the authors added.
“Limiting viral evolution and preventing further vaccine-escaping variant emergence is another fundamental task for PCV2 vaccinology monitoring,” they wrote. “Also in this case, the use of a combination of different genotypes is likely to be helpful. A combination would create a more heterogeneous immune response from which the escape of specific, single mutations would be less likely.”
Comparable evidence emerges for other pathogens of veterinary interest, for which the combination of vaccines based on different genotypes has been traditionally considered a highly effective way to protect animals from new emerging variants of viral infectious diseases.
Additional studies should be performed under experimental as well as field conditions, the researchers recommended. They noted particular care should be given to the analysis of viral evolution at the individual level.
To read the full study, click here.
1 Franzo G, Segalés S. Porcine Circovirus 2 Genotypes, Immunity and Vaccines: Multiple Genotypes but One Single Serotype. Pathogens. 2020;9:1049. doi:10.3390/pathogens9121049
Posted on June 22, 2021