Why do you need to shake vaccines before injection?

It just gets it all into suspension (if not the parts settle and separate) and evenly distributed if a multi dose vial (that is not as important as a single dose vial).. You can tell often looking at it if it needs mixing (not really shaking-more like agitation & inverting); drawing it up can also agitate it into solution. It could very likely have been mixed prior to coming into the room. Yes, it is still effective.