It is routine for basically all papers in medicine research to be hosted on PMC. Often they host the entire article, not just the index and abstract. The reason

It is routine for basically all papers in medicine research to be hosted on PMC. Often they host the entire article, not just the index and abstract. The reasons for this open access are complicated but boil down to certain rules from funding bodies. I have no idea if the particular Elsevier journal (in the banner at top of the page) is reputable, but the science here seems at least robust. If I wanted to discredit this paper, I would point out the authors developed an analysis technique they want to apply to other diseases/models. COVID is a nail and they have a specific hammer, no matter if it’s the right kind of hammer.