"Less than half of all studies, and about 60% of randomized or controlled clinical trials,initially presented as summaries or abstracts at professional meetings are subsequently published as peer-reviewed journal articles."¹
¹From the Plain Language Summary of: Scherer RW, Langenberg P, von Elm E. Full publication of results initially presented in abstracts. Cochrane Database of Systematic Reviews 2007, Issue 2. Art. No.: MR000005. DOI: 10.1002/14651858.MR000005.pub3.
Many conference proceedings, meeting abstracts, and symposia are published in journal supplements. These supplements are generally not indexed in PubMed so you need to search other places to find them.