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, >= 65 years) adults. Participants were randomized 1:4:4:4 to receive 2 doses of placebo or 7.5, 15, or 30 mu g 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, >= 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 >= 1:40. A single 7.5-mu g dose induced HI titers >= 1:40 in 94.5% (95% confidence interval [CI], 91.8%-96.3%) of all adults. After one 7.5-mg 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-mu g dose of a monovalent unadjuvanted H1N1 vaccine induced protective HI antibody levels in adults of all ages, including very elderly adults.