The Evidence-Based Metaphor "Grace and I are trying to keep the vaccines minimal for Annie, if we can."Jeremy, the man sitting in front of me, is tall, slender, and politely tattooed. Despite appearing distinctly well rested, he's every bit the new parent: exhilarated and, equally, terrified."There are just so many of them," he says. "I was concerned about overloading her system."It's a situation that many pediatricians encounter on a regular basis: a parent who is resistant to the idea of childhood vaccinations for a son or daughter. The only difference here is that Jeremy isn't really an anxious parent but a standardized patient-an actor trained to re-create this scenario-and I'm not a physician but a medical student. We're in the midst of an OSCE, an "objective structured clinical examination," designed to train medical students in the real-life practice of medicine.Sitting in a perfect replica of an examination room, wearing a white coat and stethoscope, I'm tasked with changing Jeremy's mind. I launch into my carefully prepared talking points, explaining that vaccines are remarkably safe and effective, that they won't overstrain his daughter's immune system. The vaccine we're discussing doesn't even contain living virus, I tell him. "It's more like a fingerprint of the germ," I explain. "When Annie's body sees it, she learns to recognize that tiny fragment so she can attack it if she ever encounters it again."Later, in the debriefing, Jeremy reviews the case with me, providing lengthy feedback on everything from eye contact and posture to professionalism."I've done this scenario hundreds of times," he says, finally, "and I've heard a lot of explanations of how vaccines work. I thought the fingerprint analogy worked well."This statement came almost as an afterthought, a high note to close the encounter. Yet as I left the examination, I began to wonder about the hundreds of other medical students stretching back through the years, each armed with their own individual script, each trying to accomplish the same task with different metaphors. Just how dissimilar were our explanations?Curious, I asked around. One of my classmates had described the vaccine as a "personal trainer" for the immune system, "pumping up" the patient's natural immunity. Another portrayed vaccination as a kind of insurance policy against future illness. In a case where we had all carefully memorized the same statistics, cellular pathways, and adverse effects, it occurred to me that our patient explanations seemed wildly, and perhaps unwisely, variable.Metaphor and analogy are necessary mainstays in medicine. Starting in medical school-where we learn to recognize strawberry tongues, barking seal coughs, and