It would not have a direct impact via primary app as it would be completed. It may have direct impact via the secondary app as some will not have been submitted by then. It can have some impact for schools that accept/recommend updates. However, my general view is that unless it is something prestigious, in depth, notable, significant or somehow "connected" to other things that you have done previously (directly or conceptually) , a "one-off" summer research (or any other summer "one-off" activity) may not have all that much impact. Why? Medical schools want to see a strong pattern of motivation, commitment and achievement. A "one-off" doesnt add to the pattern. It can look like last minute padding (more below on that). But it is up to you to show that pattern in your PS and secondaries.
For example, having a NOAA summer internship research cruise studying some marine ecosystem would be notable. Having taken a few oceanography courses would make that a pattern. A position in an NIH lab would notable. Working as a summer intern in a campus professor's own lab is "standard." Having been a biochem major with multiple courses in the field would make it more of a pattern. So connect it with previous work, behavior, etc. either directly or conceptually. So how much you make of it in secondary and updates can rely on the pattern it adds to.
BTW, the reason I make this point is more for those applicants who rush to do some sort of volunteering or shadowing the summer while applying . It just comes off as padding. This is much less likely in a research spot. But if you have no previous research or are not a science major, you need to at least make "a case" for it. That is, have at least a 1 or 2 line "context" and/or spin when you update. Example might be, "As a history major with only the prerequisite courses, I felt that getting exposure to biomedical research was an important aspect of my premedical education." Of course, you may simply have a wonderful opportunity show up and take it. That is a reasonable update as well though with limited impact.