Colin L. Powell, the former secretary of state and chairman of the Joint Chiefs of Staff who died Monday of what his family described as complications from covid-19, had been treated for the blood cancer known as multiple myeloma and was due to get a coronavirus booster shot when he suddenly became ill and was hospitalized, his longtime assistant, Peggy Cifrino, said Monday.

Powell, who was 84, received his second Pfizer shot in February but was immunocompromised as a result of his cancer and suffered from Parkinson’s disease, Cifrino said in an interview. Multiple myeloma is a blood cancer that severely impairs the immune system, lowering the effectiveness of vaccines.

“He was actually scheduled to receive his booster when he fell ill last week,” Cifrino said. “He couldn’t go to his appointment. … He thought he was just not feeling quite right, and he went to the hospital.”

Cifrino said Powell had been successfully treated for cancer for two or three years.

“Obviously it was a factor with his compromised immune system when he got the covid,” Cifrino said.

The family and Cifrino, who has worked for Powell for 28 years, did not release details about precisely which treatments doctors employed on Powell in recent days at Walter Reed National Military Medical Center in Bethesda, Md.

The four-star general served under three Republican presidents and was also the youngest and first Black chairman of the Joint Chiefs of Staff. (Monica Rodman, Drea Cornejo/The Washington Post)

As a blood cancer patient, even one successfully treated, Powell would have become eligible for another dose of mRNA vaccine under an August recommendation from federal regulators that covered people who are immunocompromised and may not have mounted a normal immune response from the initial shots. Officials have said the extra shots should be seen as a way for these patients to complete the initial immunization.

Citing the waning of antibodies over time, federal officials have also authorized a third shot six months after the initial course of a Pfizer vaccine for people over 65 or at higher risk of complications because of underlying health conditions or occupational exposure. A Food and Drug Administration advisory panel has recommended the same for recipients of the Moderna vaccine, though the agency must authorize the extra shot. The panel also recommended a second shot for all recipients of the single-shot Johnson & Johnson vaccine, which the agency must also authorize.

At 84, Powell was at an age already at high risk of severe outcomes from covid-19. People in that age cohort are also at higher risk of a severe outcome if they have a post-vaccination breakthrough infection.

The evidence is overwhelming that vaccines are effective at lowering the risk of hospitalization and death from covid-19. They do not, however, create an impermeable barrier to infection.

The Centers for Disease Control and Prevention has tracked 7,178 deaths among people who were fully vaccinated. Of those, 85 percent were over 65. The agency also has tracked nearly 25,000 nonfatal breakthrough infections requiring hospitalization. Men are also at higher risk, comprising 57 percent of the fatal breakthrough cases.

Albert Ko, an infectious-disease physician at the Yale Schools of Public Health and Medicine, said Monday that multiple myeloma takes over the bone marrow and impairs the ability to block infections generally.

“Bottom line, he had an unfortunate, specific type of malignancy,” said Ko, who was not part of Powell’s medical team. “He was at high risk not only for covid but a lot of different infections.”

A study published in July showed that about one in four patients with blood cancers produced no detectable antibodies after vaccination with the Pfizer-BioNTech and Moderna vaccines, though the results varied depending on which type of cancer the patient had.

Conducted by the Leukemia & Lymphoma Society and published in the journal Cancer Cell, the study and others demonstrate why immunocompromised patients are so vulnerable to the coronavirus. Antibodies provide the body’s natural defense against invaders such as the virus that causes covid-19; without them, it is difficult to ward off the disease.

Another study that focused specifically on 103 multiple myeloma patients showed that 45 percent developed an adequate antibody response to vaccination, and 22 percent had a partial response to the two companies’ mRNA vaccines. That research, led by Samuel Stampfer of the Emory University School of Medicine, was published in the journal Leukemia.

A small study released last week showed that booster shots of those vaccines produced some antibodies in about half of the patients who previously hadn’t developed any after their initial vaccination. But that research, also conducted by the Leukemia & Lymphoma Society, looked at just 49 people who had blood cancers such as multiple myeloma and chronic lymphocytic leukemia.

Samir Parekh, a hematologist and oncologist at the Icahn School of Medicine at Mount Sinai, said both multiple myeloma and the most common treatments for the disease kill the body’s antibody producing B cells.

“The factory of antibodies is not working the way it should” in patients under treatment for the disease, Parekh said.

Before the pandemic, doctors were aware that patients with this type of cancer were vulnerable to a wide range of infections, including bacterial infections and the flu, he said. It is critical for them to keep up with flu and pneumonia vaccines, and many are given monthly infusions of antibodies.

In an early look at some of his patients, booster shots of vaccine appeared to be effective at producing antibodies for those who hadn’t developed them during initial vaccination, Parekh said.

Gwen Nichols, chief medical officer for the Leukemia & Lymphoma Society, estimated that 1.3 million people are living with blood cancers or are survivors of the disease.

“We are advising them to get vaccinated but act unvaccinated,” Nichols said. “Avoid really crowded places. Avoid situations where you can’t be in control. Wear a mask. Wash your hands.”

The risks for such people highlight the need for everyone to be vaccinated and reduce the chances of passing on the infection, as someone did to Powell, she said.

“We hear it every day,” Nichols said. “They’re angry at people who won’t help them by getting vaccinated. They remain at risk despite doing everything right.”