The present study is designed to evaluate the reactogenicity and immunogenicity of one or two booster administrations of an influenza pandemic candidate vaccine (GSK1562902A) in adults aged between 19 and 61 years, previously vaccinated with 2 doses of a pandemic candidate vaccine. Fifty new subjects who did not participate in a primary study (106750, NCT00309634) will be recruited. This protocol posting deals with objectives & outcome measures of the booster phase. The objectives & outcome measures of the primary phase are presented in a separate protocol posting (NCT00309634)