Researchers should protect the welfare of research participants through providing methods to reduce their risk of acquiring HIV. This is especially important given that late-phase HIV vaccine trials enrol HIV-uninfected trial volunteers from high-risk populations. Current ethical guidelines may be difficult for stakeholders to implement, and we know very little about what prevention services researchers are currently providing to participants or their successes, best practices and challenges. We recommend that current normative guidance be systematically reviewed and actual practice at vaccine sites be documented. Adding new tools to the current package of prevention services will involve complex decision making with few set standards, and regulatory and scientific challenges. We recommend that stakeholders (including regulators) convene to consider standards of evidence for new tools, and that decision-making processes be explicitly documented and researched. A further critical ethical task is exploring the threshold at which adding new tools will compromise the validity of trial results.