A phase III, randomized, open-label study to assess the tolerability and immunogenicity of an H5N1 influenza vaccine administered to healthy adults with a 1-, 2-, 3-, or 6-week interval between first and second doses.

Preparedness for an H5N1 influenza pre-pandemic requires effective and well-tolerated emergency vaccination strategies that provide both pandemic strain-specific and heterologous protection. This was a pivotal study for the regulatory approval process for a candidate MF59-adjuvanted H5N1 vaccine. Its goals were to identify the preferred primary 2-dose vaccination schedule in adults and to assess whether the vaccine met European Committee for Medicinal Products for Human Use (CHMP) licensure criteria. Healthy volunteers aged 18 to 60 years received 1 of 4 randomized schedules in which the 2 doses of vaccine were separated by a 1-, 2-, 3-, or 6-week interval. Three blood samples (~20 mL(-1)) were obtained from each subject: the first sample, immediately before administration of the first dose of vaccine; the second, immediately before administration of the second dose; and the third, 21 days after administration of the second dose. Hemagglutination inhibition (HI), microneutralization (MN), and single radial hemolysis (SRH) were assayed after each dose. Immunogenicity was assessed based on the CHMP licensure criteria for annual influenza vaccines (number of seroconversions or significant increase in HI titer >40%; mean geometric increase >2.5; and proportion of subjects achieving an HI titer ≥40 or SRH titer >25 mm(2) should be >70% [seroprotection]). Subjects recorded all adverse events occurring within 7 days of vaccine administration; information on any serious adverse events was collected throughout the study (duration, 202 days). All study participants (N = 240) were white, with a mean age of 33 years and a mean body mass index of 24.6 kg/m(2). Equal numbers of men and women were assigned to each vaccination schedule. The CHMP criterion for seroprotection was achieved when the 2 doses of vaccine were separated by 2 (76%), 3 (72%), and 6 (79%) weeks; similar results were obtained on MN and SRH analysis. On the SRH analysis, the candidate vaccine showed a heterologous immune response to the H5N1/turkey/Turkey/1/05 (NIBRG-23; clade 2) influenza antigen. The vaccine met 2 of the 3 European licensure criteria, with seroconversion rates of 69% and 65% in the groups assigned to a 2- and 3-week interval between doses, respectively, and geometric mean ratios of 4.3 and 4.5. There were no serious adverse events related to vaccination. The most common adverse events reported within 7 days of the first and second doses of vaccine were mild to moderate injection-site pain (63%-73% and 34%-48%, respectively) and fatigue (25%-30% and 13%-24%). Two 7.5-μg doses of MF59-adjuvanted H5N1 influenza vaccine given 2, 3, or 6 weeks apart afforded H5N1-specific immunity and met the CHMP licensure criterion for seroprotection in these healthy volunteers. Clinically relevant levels of heterologous immunity were observed when the 2 doses of vaccine were administered either 2 or 3 weeks apart; however, the licensure criterion for seroprotection was not met in this case.