Like his friends, Dr. Wollowitz was vaccinated in January. But he did not produce any antibodies in response — nor did he expect to. He is one of millions of Americans who are immunocompromised, whose bodies cannot learn to deploy immune fighters against the virus. Some immunocompromised people were born with absent or faulty immune systems, while others, like Dr. Wollowitz, have diseases or have received therapies that wiped out their immune defenses. Many of them produce few to no antibodies in response to a vaccine or an infection, leaving them susceptible to the virus. When they do become infected, they may suffer prolonged illness, with death rates as high as 55 percent. Most people who have lived with immune deficiencies for a long time are likely to be aware of their vulnerability. But others have no idea that medications may have put them at risk.