The purpose of this follow-up study is to evaluate the persistence of antibodies against all the vaccine antigens 10 years after booster vaccination with either Tdap or Td, and also to assess immunogenicity and safety of another dose of Boostrix, administered in this study. This protocol posting deals with objectives and outcome measures of the extension phase. The objectives and outcome measures of the primary phase are presented in a separate protocol posting (NCT number = NCT00109330).