The 7 month visit is the final assessment included in the PREV-IX_COMBO trial. In October 2011 the standard PCV schedule for all Indigenous babies in the NT switched from PHiD-CV10 to PCV13 at 2, 4, 6 and 18 months of age. Thus, all babies in PREV-IX_COMBO will go on to receive PCV13 at 18 months of age as a part of standard care. An alternative and innovative option is to take this opportunity to answer these questions: should PCV13 or PHiD-CV10 vaccine be the booster vaccine for infants in the three PREV-IX_COMBO groups, and at what age should the booster be given? Secondary questions: By randomising the three different primary course groups to booster dose of either PHiD-CV10 or PCV13, a number of secondary questions can be proposed. Outputs include the comparative benefit (immune and carriage correlates) of schedules such as 3+1 PCV13, 3+1 PHiD-CV10, and mixed vaccine schedules such as single dose PCV13 or single dose PHiD-CV10 boosters following alternate vaccines in the primary course, and booster options following the early and mixed 4-dose primary course. Furthermore the design will allow for an evaluation at 18 months of age, of a 13 month boost versus no boost of specific vaccine components unique to each of these vaccines. For example, a comparison of the booster effect of HiD (in PHiD-CV10) with no HiD boost (PCV13) on antibody concentration and nasopharyngeal carriage at 18 months. By combining some of these groups it will be possible to compare more subtle differences such as differences in immunogenicity of serotypes shared by the two vaccines (1,4,5,6B,7F,9V,14,18C,19F,23F) but with different carrier proteins and thus anticipated differences in immunogenicity.