Background: The recent global A/H1N1v pandemic led to major efforts to develop effective vaccines against the novel virus, while global demand and limited production capacity focused attention on dose sparing and schedules.
Methods: An open-label phase III study of immunogenicity and safety of novel A/H1N1v vaccines included 392 Costa Rican children in two pediatric cohorts (3-8 and 9-17 years). They received two doses, of either an MF59®-adjuvanted formulation containing 7.5 μg antigen or non-adjuvanted formulations containing 15 or 30 μg antigen, three weeks apart. Immunogenicity was assessed as hemagglutination inhibition (HI) titers using the CBER licensure criteria.
Results: All three vaccines elicited immune responses in 9-17 year-olds meeting CBER criteria three weeks after one dose; responses were not enhanced by second dose. In 3-8 year-olds only the adjuvanted vaccine met the CBER criteria after one dose, but all three vaccines met criteria after second dose. All vaccines were well tolerated; no related Serious Adverse Events (SAE) and few severe solicited reactions were reported. MF59-adjuvanted vaccine was associated with more reports of injection site pain and tenderness and overall systemic solicited reactions, most notably in older subjects, all of which decreased after the second dose.
Conclusion: One dose of non-adjuvanted A/H1N1v vaccine is adequate in 9-17 year-olds, but younger children require either one dose of MF59-adjuvanted vaccine or two doses of non-adjuvanted vaccine to achieve protective titers. Enhanced immunogenicity with MF59 is associated with a small increase in reactogenicity, but no safety issues.