Immunogenicity of a reduced dose of A/H3N2 in the 2005 southern hemisphere formulation of inactivated split influenza vaccine

Background The 2005 southern hemisphere formulation of the inactivated split‐virion influenza vaccine Vaxigrip(®) unintentionally contained a lower concentration of haemagglutinin (HA) than European Pharmacopoeia (EP) and WHO specifications for one of the three strains. Objectives To evaluate the immunogenicity of the 2005 southern hemisphere formulation of an influenza vaccine containing 9 μg/dose of HA for A/Wellington/1/2004(H3N2) strain, and 15 μg/dose for each of the A/New Caledonia/20/99(H1N1) strain and B/Shanghai/361/2002‐like strains. Patients/methods In an open, non‐controlled multicentre clinical trial, 75 healthy adults (18–59 years) and 65 healthy older adults (≥60 years) were vaccinated once. Serum samples were obtained on D0 and 21 for haemagglutination inhibition (HAI) antibody titration. Results A high proportion of adults (64%) and elderly (68%) were already seroprotected (HAI titre of ≥40) against A/Wellington/1/2004(H3N2) before vaccination, probably due to high circulation of an antigenically similar H3N2 strain and a high 2004 vaccination rate. By D21, seroprotection rates against H3N2 attained 93·8% and 96·0% in adults and elderly respectively. The other two immunogenicity criteria for annual licensure of influenza vaccines in Europe were also met in both age groups for the H3N2 strain, and also for the H1N1 and B strains. Conclusions These results enabled the 2005 southern hemisphere vaccine to be used in expectation that it would provide satisfactory protection against influenza, despite the reduced H3N2 antigen content.