For more than a year, Dr. Andrew Wollowitz has mostly been cloistered inside his home in Mamaroneck, N.Y.

As chief of emergency medicine at Montefiore Medical Center in the Bronx, Wollowitz, 63, was eager to help treat patients when the coronavirus began raging through the city last spring. But a cancer treatment in 2019 had obliterated his immune cells, leaving him defenseless against the virus.

A year later, people in Wollowitz's life are returning to some semblance of normalcy. His wife, a dancer and choreographer, is preparing to travel for work at Austria's National Ballet Company. His vaccinated friends are getting together.

Like his friends, 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 people were born with absent or faulty immune systems, while others, like 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. When they do become infected, they may suffer prolonged illness, with death rates as high as 55%.

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.

"They'll be walking around outside thinking they're protected — but maybe they're not," said Dr. Lee Greenberger, chief scientific officer of the Leukemia and Lymphoma Society.

The only recourse for these patients — apart from sheltering in place — may be to receive infusions of monoclonal antibodies, which are mass-produced copies of antibodies obtained from people who have recovered from COVID-19.

Convalescent plasma or gamma globulin — antibodies distilled from the blood of healthy donors — may also help immunocompromised people, although a version of the latter that includes antibodies to the coronavirus is still months from availability.

It's unclear how many immunocompromised people don't respond to coronavirus vaccines. But the list seems at least to include survivors of blood cancers, organ transplant recipients, and anyone who takes the widely used drug Rituxan, or the cancer drugs Gazyva or Imbruvica — all of which kill or block B cells, the immune cells that churn out antibodies — or Remicade, a drug for irritable bowel disease. It may also include some people older than 80 whose immune responses have faltered with age.

The Leukemia and Lymphoma Society has set up a registry to provide information and antibody tests to people with blood cancers. And several studies are assessing the response to coronavirus vaccines in people with cancer, autoimmune conditions like lupus or rheumatoid arthritis, or who take drugs that mute the immune response.

In one such study, British researchers followed nearly 7,000 people with Crohn's disease or ulcerative colitis. They found that less than half of patients who took Remicade mounted an immune response following coronavirus infection.

In a follow-up, the scientists found that 34% of people taking the drug were protected after a single dose of the Pfizer vaccine and only 27% after a single dose of the AstraZeneca vaccine.

A study published last month in the journal JAMA reported that only 17% of 436 transplant recipients who got one dose of the Pfizer-BioNTech or Moderna vaccine had detectable antibodies three weeks later.

Immunocompromised people should still get the vaccines because they may produce some immune cells that are protective. "The take home message is that everybody should try and get the vaccine," said Dr. Amit Verma, an oncologist.

The gamble did not pay off in Wollowitz's case. Without antibodies to protect him, he is still working from home — a privilege he is grateful for. He said he foresaw himself living this way till enough other people are vaccinated and the number of infections drops. "I'm not exactly sure what that date is," he said. "I'm really waiting to get back out."