Development of an Influenza A Master Virus for Generating High-Growth Reassortants for A/Anhui/1/2013(H7N9) Vaccine Production in Qualified MDCK Cells

Development of an Enhanced High-Yield Influenza Vaccine Backbone in Embryonated Chicken Eggs

Vaccination is an efficient approach to preventing influenza virus infections. Recently, we developed influenza A and B virus vaccine backbones that increased the yield of several vaccine viruses in Madin-Darby canine kidney (MDCK) and African green monkey kidney (Vero) cells. These vaccine backbones also increased viral replication in embryonated chicken eggs, which are the most frequently used platform for influenza vaccine manufacturing. In this study, to further increase the viral titers in embryonated chicken eggs, we introduced random mutations into the 'internal genes' (i.e., all influenza viral genes except those encoding the hemagglutinin and neuraminidase proteins) of the influenza A virus high-yield virus backbone we developed previously. The randomly mutated viruses were sequentially passaged in embryonated chicken eggs to select variants with increased replicative ability. We identified a candidate that conferred higher influenza virus growth than the high-yield parental virus backbone. Although the observed increases in virus growth may be considered small, they are highly relevant for vaccine manufacturers.

Glycosylation generates an efficacious and immunogenic vaccine against H7N9 influenza virus

Zoonotic avian influenza viruses pose severe health threats to humans. Of several viral subtypes reported, the low pathogenic avian influenza H7N9 virus has since February 2013 caused more than 1,500 cases of human infection with an almost 40% case-fatality rate. Vaccination of poultry appears to reduce human infections. However, the emergence of highly pathogenic strains has increased concerns about H7N9 pandemics. To develop an efficacious H7N9 human vaccine, we designed vaccine viruses by changing the patterns of N-linked glycosylation (NLG) on the viral hemagglutinin (HA) protein based on evolutionary patterns of H7 HA NLG changes. Notably, a virus in which 2 NLG modifications were added to HA showed higher growth rates in cell culture and elicited more cross-reactive antibodies than did other vaccine viruses with no change in the viral antigenicity. Developed into an inactivated vaccine formulation, the vaccine virus with 2 HA NLG additions exhibited much better protective efficacy against lethal viral challenge in mice than did a vaccine candidate with wild-type (WT) HA by reducing viral replication in the lungs. In a ferret model, the 2 NLG-added vaccine viruses also induced hemagglutination-inhibiting antibodies and significantly suppressed viral replication in the upper and lower respiratory tracts compared with the WT HA vaccines. In a mode of action study, the HA NLG modification appeared to increase HA protein contents incorporated into viral particles, which would be successfully translated to improve vaccine efficacy. These results suggest the strong potential of HA NLG modifications in designing avian influenza vaccines.

This study shows that changing the pattern of N-glycosylation of the pathogenic avian influenza H7N9 virus hemagglutinin protein increases the amount of hemagglutinin incorporated into the viral membrane; the candidate vaccine virus induces neutralizing antibodies and protects animal models from lethal viral challenge.

Determination of the potency of a cell-based seasonal quadrivalent influenza vaccine using a purified primary liquid standard

In Japan, the practical application of completely cell-based seasonal influenza vaccines is under consideration. Considering the good correlation between the immunogenicity of egg-based influenza vaccines and the hemagglutinin (HA) content determined by the single radial immunodiffusion (SRD) assay, we determined the potency of the first cell-based quadrivalent vaccine experimentally generated in Japan using the SRD assay in this study. A primary liquid standard (PLS) and reference antigen were generated from the purified vaccine virus, and a sheep antiserum was produced against the HA of the vaccine virus. Since the purity of the PLS affects the reliability of vaccine potency testing, the purification steps are significant. We successfully prepared a purified PLS nearly free of cell debris. The HA content in the PLS was first estimated from the total amount of viral protein and the percentage of HA content determined by SDS-PAGE analysis. The HA content in the reference antigen was calibrated to that in the PLS via the SRD assay. The vaccine potency, that is, the HA content in each vaccine, was finally measured using the corresponding reference antigen. Ultimately, the measured vaccine potency of the monovalent vaccine was similar to that of the quadrivalent vaccine.

A Phase 1 Randomized Placebo-Controlled Study to Assess the Safety, Immunogenicity and Genetic Stability of a New Potential Pandemic H7N9 Live Attenuated Influenza Vaccine in Healthy Adults

This study describes a double-blind randomized placebo-controlled phase I clinical trial in healthy adults of a new potential pandemic H7N9 live attenuated influenza vaccine (LAIV) based on the human influenza virus of Yangtze River Delta hemagglutinin lineage (ClinicalTrials.gov Identifier: NCT03739229). Two doses of H7N9 LAIV or placebo were administered intranasally to 30 and 10 subjects, respectively. The vaccine was well-tolerated and not associated with increased rates of adverse events or with any serious adverse events. Vaccine virus was detected in nasal swabs during the 6 days after vaccination or revaccination. A lower frequency of shedding was observed after the second vaccination. Twenty-five clinical viral isolates obtained after the first and second doses of vaccine retained the temperature-sensitive and cold-adapted phenotypic characteristics of LAIV. There was no confirmed transmission of the vaccine strain from vaccinees to placebo recipients. After the two H7N9 LAIV doses, an immune response was observed in 96.6% of subjects in at least one of the assays conducted.

Segment 2 from influenza A(H1N1) 2009 pandemic viruses confers temperature-sensitive haemagglutinin yield on candidate vaccine virus growth in eggs that can be epistatically complemented by PB2 701D

Candidate vaccine viruses (CVVs) for seasonal influenza A virus are made by reassortment of the antigenic virus with an egg-adapted strain, typically A/Puerto Rico/8/34 (PR8). Many 2009 A(H1N1) pandemic (pdm09) high-growth reassortants (HGRs) selected this way contain pdm09 segment 2 in addition to the antigenic genes. To investigate this, we made CVV mimics by reverse genetics (RG) that were either 6 : 2 or 5 : 3 reassortants between PR8 and two pdm09 strains, A/California/7/2009 (Cal7) and A/England/195/2009, differing in the source of segment 2. The 5 : 3 viruses replicated better in MDCK-SIAT1 cells than the 6 : 2 viruses, but the 6 : 2 CVVs gave higher haemagglutinin (HA) antigen yields from eggs. This unexpected phenomenon reflected temperature sensitivity conferred by pdm09 segment 2, as the egg HA yields of the 5 : 3 viruses improved substantially when viruses were grown at 35 °C compared with 37.5 °C, whereas the 6 : 2 virus yields did not. However, the authentic 5 : 3 pdm09 HGRs, X-179A and X-181, were not markedly temperature sensitive despite their PB1 sequences being identical to that of Cal7, suggesting compensatory mutations elsewhere in the genome. Sequence comparisons of the PR8-derived backbone genes identified polymorphisms in PB2, NP, NS1 and NS2. Of these, PB2 N701D affected the temperature dependence of viral transcription and, furthermore, improved and drastically reduced the temperature sensitivity of the HA yield from the 5 : 3 CVV mimic. We conclude that the HA yield of pdm09 CVVs can be affected by an epistatic interaction between PR8 PB2 and pdm09 PB1, but that this can be minimized by ensuring that the backbones used for vaccine manufacture in eggs contain PB2 701D.

Cell culture-derived influenza vaccines in the severe 2017-2018 epidemic season: a step towards improved influenza vaccine effectiveness

The 2017–2018 seasonal influenza epidemics were severe in the US and Australia where the A(H3N2) subtype viruses predominated. Although circulating A(H3N2) viruses did not differ antigenically from that recommended by the WHO for vaccine production, overall interim vaccine effectiveness estimates were below historic averages (33%) for A(H3N2) viruses. The majority (US) or all (Australian) vaccine doses contained multiple amino-acid changes in the hemagglutinin protein, resulting from the necessary adaptation of the virus to embryonated hen’s eggs used for most vaccine manufacturing. Previous reports have suggested a potential negative impact of egg-driven substitutions on vaccine performance. With BARDA support, two vaccines licensed in the US are produced in cell culture: recombinant influenza vaccine (RIV, Flublok™) manufactured in insect cells and inactivated mammalian cell-grown vaccine (ccIIV, Flucelvax™). Quadrivalent ccIIV (ccIIV4) vaccine for the 2017–2018 influenza season was produced using an A(H3N2) seed virus propagated exclusively in cell culture and therefore lacking egg adaptative changes. Sufficient ccIIV doses were distributed (but not RIV doses) to enable preliminary estimates of its higher effectiveness relative to the traditional egg-based vaccines, with study details pending. The increased availability of comparative product-specific vaccine effectiveness estimates for cell-based and egg-based vaccines may provide critical clues to inform vaccine product improvements moving forward.

Isolation of an Egg-Adapted Influenza A(H3N2) Virus without Amino Acid Substitutions at the Antigenic Sites of Its Hemagglutinin

Antigenic changes in the hemagglutinin protein of recent A(H3N2) viruses often arise when these viruses adapt to their egg host. By serial egg passages of a cell-propagated virus, we successfully isolated an egg-adapted influenza A(H3N2) virus, A/Saitama/103/2014, without amino acid substitutions at the antigenic sites of its hemagglutinin protein but with multiple substitutions in its neuraminidase protein. Antigenic analysis of this egg-adapted A/Saitama/103/2014 virus indicated that its antigenicity did not differ from that of the World Health Organization prototype cell-propagated vaccine virus: A/Hong Kong/4801/2014. Our results suggest that this strategy may facilitate egg-based vaccine production without antigenic alterations in hemagglutinin by egg adaptation.

Tackling a novel lethal virus: a focus on H7N9 vaccine development

INTRODUCTION

Avian-origin H7N9 influenza viruses first detected in humans in China in 2013 continue to cause severe human infections with a mortality rate close to 40%. These viruses are acknowledged as the subtype most likely to cause the next influenza pandemic. Areas covered: Here we review published data on the development of H7N9 influenza vaccine candidates and their evaluation in preclinical and clinical trials identified on PubMed database with the term 'H7N9 influenza vaccine'. In addition, a search with the same term was done on ClinicalTrials.gov to find ongoing clinical trials with H7N9 vaccines. Expert commentary: Influenza vaccines are the most powerful tool for protecting the human population from influenza infections, both seasonal and pandemic. During the past four years, a large number of promising H7N9 influenza vaccine candidates have been generated using traditional and advanced gene engineering techniques. In addition, with the support of WHO's GAP program, influenza vaccine production capacities have been established in a number of vulnerable low- and middle-income countries with a high population density, allowing the countries to be independent of vaccine supply from high-income countries. Overall, it is believed that the world is now well prepared for a possible H7N9 influenza pandemic.