Nantucket’s only hospital is built for basic care: routine outpatient visits, a broken arm, a birth. If a patient comes in with something more serious, they board a helicopter bound for Massachusetts General Hospital in Boston, where some of the best doctors in the world will be waiting to receive them. But that door will likely close in the thick of a pandemic: Even if the hospital had the capacity to airlift everyone, Shaw said, he suspects the Boston facilities will be too full to take them in.