Assessment of the safety and efficacy of an attenuated live vaccine based on highly virulent genotype 2b porcine epidemic diarrhea virus in nursing piglets

We have previously reported the generation of the attenuated KNU-141112-S DEL5/ORF3 virus by continuous propagation of highly virulent G2b porcine epidemic diarrhea virus (PEDV) in Vero cells. The present study aimed to assess the safety of S DEL5/ORF3 and to evaluate its effectiveness as a live vaccine for prime-booster vaccinations. Reversion to virulence experiments revealed that the S DEL5/ORF3 strain retains its attenuated phenotype and genetic stability after five successive passages in susceptible piglets. Pregnant sows were primed orally with an S DEL5/ORF3 live vaccine and boosted intramuscularly twice with a commercial killed vaccine at 2-week intervals prior to parturition. This sow vaccination regimen completely protected nursing piglets against virulent G2b challenge, as evidenced by the increase in survival rate from 0% to 100% and the significant reduction in diarrhea intensity, including the amount and duration of PEDV fecal shedding. In addition, despite a 2-3 day period of weight loss in piglets from vaccinated sows after challenge, their daily weight gain was recovered at 7 days post-challenge and became similar to that of unchallenged pigs from unvaccinated sows over the course of the experiment. Furthermore, strong antibody responses to PEDV were verified in the sera and colostrum of immunized sows with the prime-boost treatment and their offspring. Altogether, our data demonstrated that the attenuated S DEL5/ORF3 strain guarantees the safety to host animals with no reversion to virulence and is suitable as an effective primary live vaccine providing durable maternal lactogenic immunity for passive piglet protection.

Methods to Monitor Molecular Consistency of Oral Polio Vaccine

Replication of viruses leads to emergence of mutations and their content in viral populations can increase by selection depending on growth conditions. Some of these mutations have deleterious effect on vaccine safety, such as neurovirulent reversions in the 5'-UTR of attenuated Sabin strains of poliovirus. Their content in vaccine batches must be tightly controlled during vaccine manufacture to ensure safety of the product. This chapter describes a quantitative molecular procedure called mutant analysis by PCR and restriction enzyme cleavage (MAPREC) that is used to monitor content of neurovirulent revertants in Oral Polio Vaccine (OPV). The method can be used for quantitative analysis of any other mutation in a viral population.

Subpopulations in aMPV vaccines are unlikely to be the only cause of reversion to virulence

Avian metapneumovirus (aMPV) infects respiratory and reproductive tracts of domestic poultry, often involving secondary infections, and leads to serious economic losses in most parts of the world. While in general disease is effectively controlled by live vaccines, reversion to virulence of those vaccines has been demonstrated on several occasions. Consensus sequence mutations involved in the process have been identified in more than one instance. In one previous subtype A aMPV candidate vaccine study, small subpopulations were implicated. In the current study, the presence of subpopulations in a subtype B vaccine was investigated by deep sequencing. Of the 19 positions where vaccine (strain VCO3/50) and progenitor (strain VCO3/60616) consensus sequences differed, subpopulations were found to have sequence matching progenitor sequence in 4 positions. However none of these mutations occurred in a virulent revertant of that vaccine, thereby demonstrating that the majority progenitor virus population had not survived the attenuation process, hence was not obviously involved in any return to virulence. However within the vaccine, a single nucleotide variation was found which agreed with consensus sequence of a derived virulent revertant virus, hence this and other undetected, potentially virulent subpopulations, can be involved in reversion. Much deeper sequencing of progenitor, vaccine and revertant may clarify whether problematic virulent subpopulations are present and therefore whether these need to be routinely removed during aMPV vaccine preparation prior to registration and release.