The hospital said it was following policy.
Brigham and Women's Hospital told the BBC in a statement: "Given the shortage of available organs, we do everything we can to ensure that a patient who receives a transplanted organ has the greatest chance of survival."
A spokesman said the hospital requires "the Covid-19 vaccine, and lifestyle behaviours for transplant candidates to create both the best chance for a successful operation and to optimise the patient's survival after transplantation, given that their immune system is drastically suppressed".
The hospital's carefully worded statement may suggest other factors lie beyond the patient's unvaccinated status for his ineligibility, but it refused to discuss specifics, citing patient privacy.
The hospital added that most of the 100,000 people on waitlists for organ transplants will not receive an organ within five years because of the shortage of available organs.