Response after One Dose of a Monovalent Influenza A (H1N1) 2009 Vaccine — Preliminary Report

Abstract

Abstract Background A novel influenza A (H1N1) 2009 virus is responsible for the first influenza pan-demic in 41 years. A safe and effective vaccine is urgently 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. Methods This preliminary report evaluates the immunogenicity and safety of the vaccine 21 days after the first of two scheduled doses. A total of 240 subjects, equally divided into two age groups (<50 years and ≥50 years), were enrolled and underwent ran -domization to receive either 15 µg or 30 µg of hemagglutinin antigen by intramus -cular injection. We measured antibody titers using hemagglutination-inhibition and microneutralization assays at baseline and 21 days after vaccination. The co-primary immunogenicity end points were the proportion of subjects with antibody titers of 1:40 or more on hemagglutination-inhibition assay, the proportion of sub -jects with either seroconversion or a significant increase in antibody titer, and the factor increase in the geometric mean titer.