The real question here is: do you have a fundamental role on the projects or are you simply running experiments while others do the thinking?
From the information in your posts thus far, there seems to be no evidence of thinking of the research question, forming hypotheses, designing approaches to answer these questions, analyzing the data, writing manuscripts, etc that would indicate a role above and beyond that of a technician.
MD/PhD admissions committees look for applicants that have had a significant research experience, meaning that you have a large role on a project, understand what you are doing, and are more involved on it than simply running experiments.
Being on a publication is not sufficient for MD/PhD admissions. Committees know that people are often added on a co-author list for contributing very little to the ideas and experiments behind the paper. In fact, it is BETTER to have an in-depth research experience in which you work on your own project, thinking about the questions you are trying to answer, designing experiments, doing the analysis and presenting the results. I didn't have any publications when I applied, but had my own independent projects that I knew inside-out and had presented at conferences and in abstracts.
Hope this helps.