Harvard University Health Services Director Giang T. Nguyen said the University waited until doses were readily accessible across the country before joining other Boston-area colleges in mandating Covid-19 vaccinations for students before returning to campus in the fall in a Thursday interview. Harvard announced the vaccine mandate on Wednesday, two weeks after all people 16 years and older became eligible for the coronavirus vaccine in every state. Massachusetts was one of the last six states to open adult eligibility by the April 19 deadline set by President Joe Biden. Though other colleges issued vaccine requirements “fairly early,” Nguyen noted that college students were not yet eligible to receive the vaccine in every state by then.