Online Ivy League degree programs that require less than a week on campus

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Technically, no one gets visas for visiting. You could potentially get a visa to study though for the summer. Harvard's Summer School program is open but of course you have to be fully enrolled in a program and be taking credits on campus to qualify. The Harvard Summer School website has the details for summer term.

This is a bit off-topic but there's an update re Harvard Summer School
Harvard Faculty to Consider Granting College Credit for Remote Summer School Courses | News | The Harvard Crimson (thecrimson.com)