How much does it help to get into med school when an applicant have published a decent scientific journal although he/she is not the first author?
Does it weigh more than volunteering at a hospital for a year?
It would be a nice addition to your application and it certainly should be listed.
As to whether it counts more than volunteering...I don't know that it does. Med schools, and especially DO schools, like to see some type of patient contact through volunteering, shadowing Drs, job, etc so the volunteering would actually be quite important. The article is very nice, but not essential.
On the other hand, if you're applying DO/PhD the article would be quite important, but the volunteering/etc would still be impt too.