A single dose of unadjuvanted novel 2009 H1N1 vaccine is immunogenic and well tolerated in young and elderly adults.

BACKGROUND When the novel H1N1 influenza A strain appeared in April of 2009, development of novel H1N1 vaccines became a public health priority. METHODS We conducted a phase‐2, multicenter, randomized, placebo‐controlled, observer‐blind clinical trial of a 2009 H1N1 vaccine in 1313 young (age, 18-64 years) and older (age, >or=65 years) adults. Participants were randomized 1:4:4:4 to receive 2 doses of placebo or 7.5, 15, or 30 μg of H1N1 hemagglutinin administered 21 days apart. In post hoc analyses, hemagglutination inhibition (HI) titers measured at baseline and after vaccination were analyzed for young adults (age, 18-64 years), "younger elderly" adults (age, 65-74 years), and "very elderly" adults (age, >or=75 years). RESULTS At baseline, 28.8% of young adults, 43.9% of younger elderly adults, and 62.9% of very elderly adults had HI titers to A/2009 H1N1 of >or=1:40. A single 7.5‐μg dose induced HI titers >or=1:40 in 94.5% (95% confidence interval [CI], 91.8%-96.3%) of all adults. After one 7.5‐μg dose, the geometric mean titers achieved were 326.4 (95% CI, 275.9-386.0) in young adults, 155.4 (95% CI, 123.4-195.8) in "younger elderly" adults, and 243.9 (95% CI, 167.1-356.0) in "very elderly" adults. CONCLUSIONS This large phase-2 trial demonstrated that a single 7.5‐μg dose of a monovalent unadjuvanted H1N1 vaccine induced protective HI antibody levels in adults of all ages, including very elderly adults. TRIAL REGISTRATION Clinicaltrials.gov identifier NCT00958126.