[METHODS] In this randomized , observer-blinded study , adults 18 years of age ( n = 841 ) received 3.75 or 7.5 g hemagglutinin antigen ( HA ) of an AS03-adjuvanted ( AS03A or AS03B ) A/Indonesia/5 / 2005 H5N1 ( subclade 2.1 ) vaccine ( priming ) , followed by the same HA dose of AS03-adjuvanted A/turkey/Turkey / 1/05 H5N1 ( clade 2.2 ) influenza vaccine as a booster 6 or 18 months after priming ; an unprimed group received placebo at Day 0 , and 3.75 g HA of AS03A-adjuvanted booster vaccine at 6 and 18 months .