Influenza vaccine effectiveness varies from year to year and is generally poorest against A(H3N2), the virus subtype for which genetic and antigenic evolution among circulating strains has been greatest. While vaccination may stimulate robust antibody responses to vaccine antigen, the breadth of antibodies generated may be insufficient to protect vaccinees from infection by all circulating viruses. Furthermore, vaccine-induced antibody responses may become blunted after repeated vaccinations. However, investigations to-date have largely only examined antibody titres against the vaccine antigen, and have mostly not considered vaccination history.

In recent years, problems associated with egg-based production of vaccine strains have exacerbated these problems. Influenza viruses generally acquire substitutions within the hemagglutinin (HA) protein to adapt to growth in eggs. In the case of A(H3N2) viruses these adaptations often render them antigenically distinct from the wildtype virus. Subsequently, antibodies induced against egg-adapted epitopes in the vaccine will provide limited protection against infection by circulating viruses, and vaccine effectiveness has been very low. Cell-based vaccines have been developed which can overcome some of the problems associated with egg manufacturing, but both egg and cell-based vaccines depend on the growth and purification of live viruses that must be inactivated and split before being formulated into vaccines. The chemical inactivation process disrupts key antigenic structures and alters vaccines' antigenicity, and are thus likely to impact vaccine efficacy. The Sanofi recombinant vaccine Flublok® uses a recombinant technology to produce purified HA in an un-cleaved form that is unable to mediate endosomal and viral membrane fusion. Importantly the manufacturing process does not require any chemical inactivation, meaning that the HA proteins are not exposed to any potential cross-linking agents that may alter antigenicity of the vaccine. In addition, Flublok® contains a higher concentration of antigen than standard-dose vaccines with 45 μg of each antigen included. A recent comparative analysis of antibody response from healthy adults (18-49 years old), comparing egg-based, cell-based and recombinant (Flublok®) vaccines found that Flublok® resulted in significantly higher titres of neutralizing antibody and that the recombinant vaccine may have properties that allow for better viral neutralisation compared to traditional cell-based vaccines. As such, clinical efficacy gains could be associated with differences between the HA in the different vaccines.

This study will assess the immunogenicity of QIV-R (Flublok) against QIV-E (Fluarix) and QIV-C (Flucelvax) vaccines, and investigate the attenuating effects of prior vaccination on vaccine immunogenicity. It will be a randomized, modified double-blind study conducted in Singapore on 360 adults, aged 21-49 years. Randomisation will be stratified by vaccination history, frequently vaccinated (3+ vaccinations during the preceding 5 years) vs. infrequently vaccinated (0-1 vaccination during the preceding 5 years), to compare the responses to each vaccine. This study is powered to primarily assess the immunogenicity (as assessed by haemagglutination inhibition (HI) geometric mean titres (GMTs) at 14-21 days post-vaccination) of QIV-R compared with QIV-E and QIV-C. Pre-vaccination, post-vaccination and post-season serum samples will be tested for antibody titres against the 4 vaccine strains in QIV-R, QIV-C and QIV-E vaccines via HI assays. For some A(H3N2) and B viruses, a microneutralisation (MN) assay will be used to assess the ability of antibodies to neutralize virus infectivity. Over the 1 year follow up period, participants who report acute respiratory infection (ARI) symptoms will have their respiratory swabs collected and tested for Influenza using reverse transcription real-time polymerase chain reaction (RT-PCR). Influenza-positive samples will be forwarded to the WHOCCRRI for virus characterization. The virus subtype (for influenza A) or lineage (for influenza B) will be identified. Viruses will be isolated and tested by HI/MN or similar assay to assess antigenic match to vaccine, and sequenced to assess genetic match to the vaccine and to identify any genetic clusters. Peripheral blood mononuclear cells (PBMCs) will be stained with up to four fluorescent labelled recombinant HA probes representing the vaccine strain and prior A(H3N2) vaccine strains, together with monoclonal antibodies against B cell activation and differentiation markers and isotypes (IgG, IgG3, IgM, IgA, IgD) to compare the magnitude of total HA-reactive B cell response and HA cross-reactivity profiles.