Study Regimen/Intervention: This is a double blind RCT of two vaccines (Pneumovax®23 and Fluarix®) plus placebo (sterile saline injection). Study participants will be followed for 28 weeks after enrollment. During this 28-week period, blood and genital secretion samples will be collected on day 0 and five subsequent time points after each injection (days 2, 4, 7, 14 and 30). Injections (vaccine or placebo) will be administered 12 weeks apart and in a random order, to minimize a possible bias due to the order of the vaccines.