Is having outside extracurricular activities going to help boost my chances to get accepted?

so first, you've got a long time before you'll be applying for medical school (at Harvard or anywhere else), because that would come after you've completed both high school and whatever undergraduate college you end up going to. all medical programs are graduate programs that you enter after finishing a Bachelor's degree, and they will really only care about specific things that you do while you're in college—medical shadowing and volunteering, taking a specific set of courses, and scoring well on the MCAT

now, if you're talking about studying somewhere for undergrad and being a pre-med (which means you're taking the courses that would put you on track for applying to medical school afterwards), then yes joining outside clubs will help you. most colleges will look for students who have been involved in a lot of organizations and extracurriculars, especially if you've gained some leadership responsibilities in them.

Yep, the other answer and comment is spot on. Med school is for after you already have a Bachelors degree in which case a school isn't going to be particularly interested in what you did in high school. If you're hoping to go down a pre-med route during undergrad then outside ECs will help boost your chances. You have to remember though that almost every other student applying to anywhere you apply is going to also have ECs (both from their schools and outside of them). So for your EC activities to give you the "boost" you're probably looking for you're going to need some very solid ECs. The good news is you have plenty of time to work on that these next 2 years and you have even more time before applying to med school.