First off, one EC is not better or more significant than any other EC. Shadowing a physician is no different than volunteering at a hospital, or playing an instrument in the band, performing on stage, or competing on an athletic field, writing for your student newspaper, being a member of the debate team, robotics club or participating in ANY other activity.
That said, Extracurricular activities come into play several different ways:
When an Admissions Offer looks at your file, they are looking to see whether you’ve made a long-time commitment (measured in years, not months) to something outside of the classroom. The idea is that a student’s commitment, drive and energy to something beyond academics is a transferable skill that might be applied to another activity in college or later in life.
Once your application has been brought to the full-committee (most applications are not) then randomness comes into play. For example, if the committee has just admitted a dozen violin players and your application comes up next, and you play the violin, the committee might put your application on hold and look for an applicant with another interest, as they don’t want to admit too many students with the same EC interest.
My gut feeling is that shadowing a doctor might be pretty common for a student interested in pursuing medicine or biomedical engineering, and as you haven’t been shadowing for very long, that activity is not going to push you into the admit pile if your application is already straddling the fence.