Similar immunogenicity and safety of the A/H1N1 2009 pandemic influenza strain when used as a monovalent or a trivalent vaccine

Background: The WHO recommended including the A (H1N1) 2009 pandemic strain in the influenza vaccines for use in the 2010-2011 northern hemisphere (NH) influenza season. The immunogenicity and safety of the trivalent split inactivated vaccine (Vaxigrip (R)) NH 2010-2011 formulation was compared to that observed for the corresponding non-adjuvanted monovalent A (H1N1) pandemic vaccine (Panenza (R)), when tested in similar populations of adult and elderly volunteers. Methods: The monovalent vaccine was evaluated in two clinical trials, conducted respectively in both adult and elderly subjects and in a population of adults. The trivalent vaccine was evaluated in a clinical study that enrolled both adult and elderly subjects. Antibody titers were measured in serum samples drawn at day 0 (before vaccination) and 21 days after one vaccine injection using the same hemagglutination inhibition (HI) assay method. The occurrence of adverse events was reported up to 21 days after vaccination. Results: Before immunization in the three studies, most of the volunteers had antibody titers below seroprotective levels against the pandemic A(H1N1) 2009 virus. After vaccination, in each trial and in each age group, high seroprotection rates, GMT ratios and seroconversion rates were observed. Seroprotection rates after administration of the monovalent vaccine reached 93% and 98% in the adult groups and 83.7% in the elderly group. After administration of the trivalent vaccine, seroprotection rates of 92.2% and 81.3% were obtained respectively in the adult and the elderly groups. No related serious adverse events and no safety signals were detected either with the monovalent or trivalent vaccine. Conclusion: Comparable immunogenicity and safety profiles were observed in three clinical trials of the pandemic A(H1N1) 2009 strain when formulated either as a monovalent or as a component of a seasonal trivalent vaccine.