Recruitment will be guided by pre-specified pausing rules. Recruitment of Group 1 will be staggered as follows: a sentinel cohort of 2 subjects (1 subject receiving SynGEM® and 1 subject receiving placebo) will be recruited on study Day 1; subjects will be followed up to Visit 2 (3 days post-dosing) and if no pausing rule is met, a second cohort of 2 subjects (1 subject receiving SynGEM® and 1 subject receiving placebo) will be vaccinated; subjects will be followed up to Visit 2 (3 days post-dosing) and if no pausing rule is met, recruitment will be extended to the remaining 20 subjects (16 on SynGEM®/4 on placebo) of Group 1.