Phenotypic properties resulting from directed gene segment reassortment between wild-type A/Sydney/5/97 influenza virus and the live attenuated vaccine strain

Abstract

Widespread use of a live-attenuated influenza vaccine (LAIV) in the United States (licensed as FluMist) raises the possibility that vaccine viruses will contribute gene segments to the type A influenza virus gene pool. Progeny viruses possessing new genotypes might arise from genetic reassortment between circulating wild-type (wt) and vaccine strains, but it will be difficult to predict whether they will be viable or exhibit novel properties. To begin addressing these uncertainties, reverse-genetics was used to generate 34 reassortant viruses derived from wt influenza virus A/Sydney/5/97 and the corresponding live vaccine strain. The reassortants contained different combinations of vaccine and wt PB2, PB1, PA, NP, M, and NS gene segments whereas all strains encoded wt HA and NA glycoproteins. The phenotypes of the reassortant strains were compared to wt and vaccine viruses by evaluating temperature-sensitive (ts) plaque formation and replication attenuation (att) in ferrets following intranasal inoculation. The results demonstrated that the vaccine virus PB1, PB2, and NP gene segments were dominant when introduced into the wt A/Sydney/5/97 genetic background, producing recombinant viruses that expressed the ts and att phenotypes. A dominant attenuated phenotype also was evident when reassortant strains contained the vaccine M or PA gene segments, even though these polypeptides are not temperature-sensitive. Although the vaccine M and NS gene segments typically are not associated with temperature sensitivity, a number of reassortants containing these vaccine gene segments did exhibit a more restricted ts phenotype. Overall, no reassortant strains were more virulent than wt, and in fact, 33 of the 34 recombinant viruses replicated less efficiently in infected ferrets. These results suggest that genetic reassortment between wt and vaccine strains is unlikely to produce viruses having novel properties that differ substantially from either progenitor, and that the likely outcome of reassortment will be attenuated viruses.

Insertion of a multibasic cleavage site in the haemagglutinin of human influenza H3N2 virus does not increase pathogenicity in ferrets

“…Both groups of inoculated ferrets revealed no virus replication in the lower respiratory tract. This is not unusual, as strain-to-strain variability among human H3N2 viruses with respect to infection of the lower respiratory tract of ferrets has been demonstrated (Jackson et al , 2009; Parks et al , 2007; Svitek et al , 2008; Zitzow et al , 2002). Although the H3N2 MBCS virus was detected in the brain of ferrets, virus titres were very low and virus-infected cells were not detected by immunohistochemistry.…”

Influenza A Virus in Swine: Epidemiology, Challenges and Vaccination Strategies

“…LAIVs are the only vaccines that generate mucosal immunity and are a promising tool in the prevention of IAV-S, but they can replicate in the host and reassortment with field strains cannot be ignored. In fact, reassortment capabilities of LAIVs were demonstrated in experimental conditions and in the field (115,130). Although this reassortment did not result in increased virulence, further research will be needed to evaluate the impact of this process on influenza A virus's epidemiology.…”

H5N1 vaccines in humans

“…Parks et al (Parks et al, 2007) reported that reassortant viruses, generated by reverse genetics, derived from a wt H3N2 influenza virus and the corresponding LAIV, were generally no more virulent than the wt H3N2 virus in the ferret model, and, in fact, the data suggested that it is likely that reassortment between the wt and vaccine viruses would result in progeny viruses that would be attenuated compared to the wt virus. The concerns of reassortment with widespread use of a pLAIV prompted us to investigate whether reassortant viruses between candidate pLAIV viruses and contemporary human influenza viruses were viable, and, if so, whether they exhibited a growth advantage and increased virulence in animal models compared to the parent wt human influenza viruses and the candidate pLAIV.…”