- Experimental vaccine(s) received in a prior vaccine trial in the 5 years before study entry. Exceptions may be made for vaccines that are currently licensed or have subsequently undergone licensure. For potential participants who have received control/placebo in an experimental vaccine trial, the protocol safety review team (PSRT) will determine eligibility on a case-by-case basis. For potential participants who have received an experimental vaccine(s) more than 5 years ago, eligibility for enrollment will be determined by the PSRT on a case-by-case basis.