The big one at my school at least is the hepatitis b, because it’s required and many people don’t keep their childhood immunity. It also takes a few months to get reimmunized and then get a titer proving immunity. If you’re fully vaccinated as far as the normal vaccines you get during childhood, you shouldn’t really need to worry. Tdap is good for 10 years, so if you want to go get a tdap, go for it, but you should have time between acceptance and matriculation.