I'm not sure if more pre-meds are first-gen or are 1st-2nd generation immigrants. My bet is the latter is actually more common than the former.
I don't see more recent data, but per the AAMC in 2008, only 20% of fathers of med students didn't have a bachelors or higher - and 25% of mothers. In fact, over half of fathers and 35% of mothers had graduate degrees. Obviously these aren't independent stats, but certainly <20% of med students (then) were first gen college students.
Meanwhile, anecdotally, significantly more than 20% of my classmates were first or second generation immigrants. From all over the world - Asia, Europe, Latin America, Africa. 14% of Americans are immigrants overall, but it's much higher among physicians - and children of physicians are disproportionately represented in med schools.