By Dr. Christine Ritchie’s estimate, about 2 million people in the United States are homebound, and an additional 5 million have trouble leaving home or need help doing so. Yet those millions of people “tend to be sort of invisible to society,” said Ritchie, a professor at Harvard Medical School. Advertisement They can’t drive through mass vaccination sites or stand in line outside clinics. Even if they secure a coveted appointment spot, they can’t leave their house to get there. Now there are doctors and nurses racing against traffic, inclement weather and a ticking clock to get...