A novel 2009 influenza A (H1N1) virus is responsible for the first influenza pandemic in 41 years. A safe and effective vaccine is needed. A randomized, observer-blind, parallel-group trial evaluating two doses of an inactivated, split-virus 2009 H1N1 vaccine in healthy adults between the ages of 18 and 64 years is ongoing at a single site in Australia.

We evaluated the immunogenicity and safety of the vaccine after each of two scheduled doses, administered 21 days apart. A total of 240 subjects, equally divided into two age groups (<50 years and ≥50 years), were enrolled and underwent randomization to receive either 15 μg or 30 μg of hemagglutinin antigen by intramuscular injection. We measured antibody titers using hemagglutination-inhibition and microneutralization assays at baseline and 21 days after vaccination. The coprimary immunogenicity end points were the proportion of subjects with antibody titers of 1:40 or more on hemagglutination-inhibition assay, the proportion of subjects with either seroconversion or a significant increase in antibody titer, and the factor increase in the geometric mean titer.

By day 21 after the first dose, antibody titers of 1:40 or more were observed in 114 of 120 subjects (95.0%) who received the 15-μg dose and in 106 of 119 subjects (89.1%) who received the 30-μg dose. A similar result was observed after the second dose of vaccine. No deaths, serious adverse events, or adverse events of special interest were reported. Local discomfort (e.g., injection-site tenderness or pain) was reported by 56.3% of subjects, and systemic symptoms (e.g., headache) by 53.8% of subjects after each dose. Nearly all events were mild to moderate in intensity.

A single 15-μg dose of 2009 H1N1 vaccine was immunogenic in adults, with mild-to-moderate vaccine-associated reactions. (ClinicalTrials.gov number, NCT00938639.)