BACKGROUND: With the ongoing 2009 pandemic of influenza A H1N1, development of pandemic influenza vaccines has generated much interest. We investigated the safety and immunogenicity of a whole-virion, inactivated, adjuvanted pandemic H1N1 vaccine in adult and elderly volunteers, given without or simultaneously with the 2009-10 seasonal trivalent influenza vaccine. METHODS: This prospective, randomised study was undertaken in two centres in Hungary. 355 participants, including 203 adults (18-60 years) and 152 elderly people (>60 years), were assigned by stratified randomisation to either 0.5 mL of the pandemic vaccine (Fluval P, a monovalent vaccine with 6 mug haemagglutinin per 0.5 mL content and aluminium phosphate gel adjuvant; n=178) or 0.5 mL of the pandemic vaccine and 0.5 mL of the seasonal trivalent vaccine (Fluval AB, a trivalent inactivated whole-virion influenza vaccine; n=177). All vaccinations were done by specific study personnel, who did not take part in the assessment of safety or immunogenicity. Co-primary objectives were safety and immunogenicity by haemagglutinin inhibition testing. All analyses were done according to a pre-established analysis plan. This study is registered with ClinicalTrials.gov, number NCT01010893. FINDINGS: Two participants receiving the pandemic vaccine only (group 1) and one receiving pandemic and seasonal vaccines (group 2) were lost to follow-up. Participants in both groups developed antibody responses against the pandemic influenza A H1N1 virus (group 1: seroconversion for adults 74.3%, 95% CI 64-6-82.4 and for elderly people 61.3%, 49.1-72.4; group 2: 76.8%, 67.2-84.7 and 81.8%, 71.4-89.7, respectively). Single doses of 6 mug fulfilled European Union and US licensing criteria for interpandemic and pandemic influenza vaccines. Simultaneously, participants in group 2 developed the immune responses needed for licensing for all three seasonal strains in the seasonal vaccine for the 2009-10 season. All adverse events were rare, mild, and transient; the most frequent were pain at injection site (eight cases in group 1 vs 18 in group 2) and fatigue for 1-2 days after vaccination (three vs five cases). INTERPRETATION: The present pandemic vaccine is safe and immunogenic in healthy adult and elderly patients, and needs low doses and only one injection to trigger immune responses to comply with licensing criteria. It can be safely co-administered with the 2009-10 seasonal influenza vaccine.