In a phase I double-blind study ( www.clinicaltrials.gov : NCT00707798 ) , healthy adults ( 18-40 years ) were randomized ( 1:2:2:2:2:2:2 ) to receive two doses of one of six investigational vaccine formulations 2 months apart , or a single dose of the control 23-valent pneumococcal polysaccharide vaccine ( 23PPV ; Pneumovax23 , Sanofi Pasteur MSD ) followed by placebo .