Analyses of hemagglutinin (HA) specific antibody responses using different serological assays after three types of influenza vaccine in children and adults

Influenza is a contagious respiratory infection caused by a single-stranded, enveloped RNA viruses, able to cause significant morbidity and mortality all over the world, together with a significant economic burden. Vaccination strategies are the most effective methods of preventing and controlling seasonal influenza epidemics that generally occur during winter season. Every year, the composition of the vaccine has to be revaluated due to the antigenic drift mechanism inherent to the influenza viruses. In order to be licensed in the European Union and declared effective and immunogenic, every new influenza vaccine has to fulfill three criteria (at least one of the assessment should meet the requirements) (CPMP/BWP/214/96), for Haemagglutination-inhibition (HI) and/or Single Radial Haemolysis (SRH) assays. Traditionally, influenza vaccines are inactivated preparations administered as a intramuscular injection containing a standardized amount of HA influenza antigen; an alternative way of administration is the intradermal injection, evaluated in the present study. Another type of influenza vaccine, today licensed and available in Russia, U.S.A and in some European countries like Norway, is the live-attenuated influenza vaccine (LAIV), administered as a nasal spray. The actual serological assays, generally used to evaluate the immunogenicity of an influenza vaccine, are the HI and the SRH, for which correlates of protection are established. The Micro-neutralization (MN), although does not present a standardized methodology and approved correlates of protection, is the most sensitive and highest specific assay able to detect functional neutralizing antibody against the HA influenza antigen. The present work has been divided in three main tasks. The first one is based on the set-up and standardization of a seasonal influenza growth method performed in MDCK-cell culture, with a serum free medium (SF), and a new method for virus titration in MN assay. The second one is a clinical study for the evaluation of the immunogenicity and safety of a quadrivalent intradermal seasonal influenza vaccine conducted in 150 healthy adults. The third one, is a comparative analysis, carried out on a small number of samples, with the aim to assess the difference in the magnitude of HA specific IgG subclass and IgA responses in healthy adults, children (<9 and >9 years) and health care workers (HCWs) after two different types of seasonal Influenza vaccine: LAIV (Live Cold-Adapted Influenza Vaccine) and IIV (Inactivated Influenza Vaccine). The strain chosen for the purpose of the study is the A/H3N2/Texas/50/2012 seasonal influenza strain (HA1 and HA2). The intradermal quadrivalent vaccine, results to be comparable, in terms of immunogenicity, to the intramuscular vaccine, confirming the capability of the intradermal injection to elicit a proper immune response. All the CPMP criteria were met for all the four seasonal strains. Moreover positive correlations were found between the results derived by the three different serological assays able to detect different anti HA-influenza antibodies in serum samples. The LAIV vaccine was to be able to promote a stronger systemic immune response in children than in adults. In adults, TIV induces better antibody responses compared to LAIV, but comparable antibody response to that induced in LAIV vaccinated children. The different mechanism of action of LAIV versus TIV, may explain the relative efficacy between the two vaccines in children and adults. In children, the avidity of pre-existing serum antibodies plays a role in determining the antibody response to infection. The present results suggest that exposure history and the type of vaccine play a significant role in determining the antibody response.